
91 S.W.3d 396 (2002)
ALFORD CHEVROLET-GEO, et al., Appellants,
v.
Jett JONES and Envo-Tech, Inc., Appellees.
No. 06-02-00058-CV.
Court of Appeals of Texas, Texarkana.
Submitted October 1, 2002.
Decided October 25, 2002.
Rehearing Overruled December 27, 2002.
*399 Robert T. Mowrey, Thomas G. Yoxall, Locke, Liddell & Sapp, LLP, Dallas, William F. Ikard, Popp & Ikard, LLP, Austin, Suzanne H. Woods, Williams & Connolly, LLP, Washington, DC, Michael C. Holmes, Christopher W. Byrd, Vinson & Elkins, Houston, for appellant.
Robert L. Templeton, John T. Smithee, Templeton, Smithee, Hayes, Fields & Young, Amarillo, Damon Young, Young & Pickett, Texarkana, for appellee.
Before MORRISS, C.J., GRANT and ROSS, JJ.

OPINION
Opinion by Justice GRANT.
Alford Chevrolet Geo, et al. (Dealers), a group of over 600 car dealerships, appeal from an interlocutory order[1] granting class certification to a group consisting of over a million individuals who purchased cars from those dealerships after September 1, 1995.
This lawsuit was filed on behalf of a group of customers against a group of car dealerships, alleging they made misrepresentations about the nature of a tax, the dealers inventory tax, representing to consumers that it was a type of vehicle inventory tax the customer was required to pay. The customers also allege the Dealers, as a group, engaged in a civil conspiracy to force, coerce, or confuse consumers into paying a tax that was levied against and was properly payable by the Dealers.
Extensive discovery has been completed. The trial court, after a hearing, certified a class of consumers. The keystone of the lawsuit lies in the nature of the term used by each dealership to describe the tax and the allegations of an agreement among the new car dealers of the State of Texas to charge that tax to the consumer. If the term dealers inventory tax was used in a sales document, the court concluded the purchaser does not qualify as a plaintiff member of the class. If one of twenty-nine other less accurate or descriptive terms was used to describe the charge, then an individual qualifies as a member of the class. (The terms were taken from sales documents obtained during discovery.)
Standard for Interlocutory Review of Class Certification
An appellate court reviews a trial court's determination that a case should be certified as a class action using an abuse of discretion standard. W. Teleservices, Inc. v. Carney, 37 S.W.3d 36, 40 (Tex.App.-San Antonio 2000, no pet.); Health & Tennis Corp. of Am. v. Jackson, 928 S.W.2d 583, 587 (Tex.App.-San Antonio 1996, no writ). The trial court abuses its discretion when it does not properly apply the law to the undisputed facts, when it acts arbitrarily or unreasonably, or when its ruling is *400 based on factual assertions unsupported by the record. Carney, 37 S.W.3d at 40.
The Dealers' arguments center around their contention that the information provided to the trial court did not support its conclusion that common issues predominate.[2]
The arguments as set out by the Dealers focus on the question of whether the trial court abused its discretion by deciding the plaintiffs had satisfied Rule 42(b)(4). That rule requires the court to conclude that questions of law or fact common to the class predominate over questions affecting only individual members and that class treatment is superior to other available methods for the fair and efficient adjudication of the controversy. Tex.R. Civ. P. 42(b)(4); see Amchem Prods., Inc. v. Windsor, 521 U.S. 591, 615, 117 S.Ct. 2231, 138 L.Ed.2d 689 (1997); Southwestern Ref. Co. v. Bernal, 22 S.W.3d 425, 433 (Tex.2000). In making this determination, the trial court is required to perform a "rigorous analysis" before ruling on class certification to determine whether all the prerequisites to certification are met. Bernal, 22 S.W.3d at 435.[3]
This rigorous analysis includes indicating how the claims will likely be tried so that conformity with Tex.R. Civ. P. 42 can be meaningfully evaluated. Id. A court must understand the claims, defenses, relevant facts, and applicable substantive law in order to make a meaningful determination of the certification issues. Id.
Rule 42(a), (b)(4)[4]
The Dealers complain at length that the common issues do not predominate over *401 questions affecting only individual members. Specifically, they complain that proof of conspiracy will require 600 mini-trials and that each individual plaintiff will have to prove reliance separately in order to recover on each plaintiff's Deceptive Trade Practices Act (DTPA) claims. Alternatively, the Dealers argue that the defenses raised by each dealer will differ for each and every sale. Therefore, they conclude, the requirements of Rule 42(b)(4) have not been met.
Predominance of Individual Issues
In the terms of the analysis, we must apply the central theme of this appeal, which is that the trial court abused its discretion by certifying the class because common issues do not predominate as required by Rule 42(b)(4); or alternatively, that separate defensive issues would require multitudinous mini-trials.[5] We do not review the record de novo, but we must determine whether the trial court adhered to the rule's requirements in making its decision, based on the "rigorous analysis" described above.
In deciding that matter, the trial court looks to see "if common issues predominate by identifying the substantive issues of the case that will control the outcome of the litigation, assessing which issues will predominate, and determining if the predominating issues are, in fact, those common to the class." Bernal, 22 S.W.3d at 434. The test for predominance is not whether common issues outnumber uncommon issues, but "whether common or individual issues will be the object of most of the efforts of the litigants and the court." Id. "If, after common issues are resolved, presenting and resolving individual issues is likely to be an overwhelming or unmanageable task for a single jury, then common issues do not predominate." Id. "Ideally, `a judgment in favor of the class members should decisively settle the entire controversy, and all that should remain is for other members of the class to file proof of their claim.'" Id.
The predominance requirement "is intended to prevent class action litigation when the sheer complexity and diversity of the individual issues would overwhelm or confuse a jury or severely compromise a party's ability to present viable claims or defenses." Id.; Carney, 37 S.W.3d at 41.
In this case, the trial court reviewed numerous affidavits presented by the parties and heard arguments of counsel on this matter. Although appellants complain in passing that the court only spent a few hours on the certification hearing, there is no authority requiring either an unnecessarily lengthy hearing or live testimony. We find that submission of affidavits is adequate and believe that in many certification efforts of either alleged or real complexity, written materials are both more readily digested and analyzed by the court in reaching this decision. So that this may be accomplished, we also note counsel will necessarily be quite reasonably required to provide to the trial court the information they want considered early enough to allow the court to consider it. Late-filing of such documents should not occur, and the trial court would be well within its authority to refuse to consider such in making its decision.
The Findings of the Trial Court
The trial court found the following issues were predominant: (1) whether the *402 plaintiff class were consumers; (2) whether the charging of the vehicle inventory tax in addition to the sales price is a false, misleading, or deceptive act or practice; (3) whether such acts were a producing cause of damages; and (4) the amount of damages.
Conspiracy Claim
Our review of the certification is to consider as one factor the evidence produced to show that the issues involved are in common. One portion of the Dealers' argument takes the position there is nothing to show that conspiracy can lie as to all members of the class. They take the position the conspiracy cannot be proven generally, but that all elements must be proven separately as to each defendant dealer. The elements of a civil conspiracy are (1) two or more persons, (2) an object to be accomplished, (3) a meeting of the minds on the object or course of action, (4) one or more unlawful, overt acts, and (5) damages as a proximate result. Operation Rescue-Nat'l v. Planned Parenthood of Houston and Southeast Tex., Inc., 975 S.W.2d 546, 553 (Tex.1998).
As set out in the findings of the trial court, discovery shows that twenty-nine different terms were used by the defendants other than the term dealers inventory tax. The customers allege that those terms did not fully or accurately inform the customers about the nature of the tax and that those terms actively misled the customers about the nature of the tax. The customers point to discovery showing the common plan orchestrated by the Dealers' trade organizations to ensure that all dealers charged the tax in the same manner and point to language in the organizations' literature and seminars to show an absolute and clearly expressed intent to shift the cost of the tax directly to the consumer. The plaintiffs allege the Dealers acted in a substantially uniform manner and caused them damages.
A common question is raised in this case: the nature of the alleged wrongdoing and the way in which it was accomplishedthrough the shifting of a tax against the dealer to the purchaser of the vehicle. The documents and depositions obtained through discovery clearly reflect that the Dealers' trade organizations designed the mode in which the tax would be presented to the purchaser and also made it clear that the reason for doing it in this fashion was to add thirty to fifty dollars to the bottom line of every sale.
The fact that the specific term used to achieve that purpose differed in detail from dealership to dealership (which is the focus of the Dealers' arguments) is not sufficient to show that the issue is not common to all parties. This portion of the analysis finds support in the information provided to the trial court.
The Dealers also complain at length the trial court has obviously already decided the merits of the case because, in its trial plan it refers to the terminology used by the various Dealers in describing the "tax" as representing a "common thread of deceit" that will constitute class membership. The Texas Supreme Court's opinion in the initial mandamus involving this case, In re Alford Chevrolet-Geo, 997 S.W.2d 173, 183 (Tex.1999), contains specific language suggesting if discovery revealed evidence indicating that a common thread of deceit ran through the representations, that would weigh in favor of certification. The trial court did not decide the case on the merits, but only recognized there was evidence suggesting such a thread of deceit as an underpinning for its certification.
*403 Conspiracy Part II
The Dealers also argue that individual issues predominate because the purchasers would have to prove what each dealer knew about the tax: that each dealer individually understood the acts were unlawful and that each dealer had a meeting of the minds with others to accomplish the acts.
As recognized by the trial court in its trial plan, conspiracy claims are often proven not by the details that testimony shows a defendant knew, but by proof of knowledge coupled with, or as shown by, his or her action, and enjoyment of the fruits of the transactions. Further, in this case, the allegations involve acts allegedly taken in unison by an entire category of car dealers.
As this court recently recognized, it is not the agreement itself, but an injury to the plaintiff resulting from an act done pursuant to the common purpose that gives rise to a cause of action for civil conspiracy. Carroll v. Timmers Chevrolet, Inc., 592 S.W.2d 922, 925 (Tex.1979); Lesikar v. Rappeport, 33 S.W.3d 282, 301-02 (Tex.App.-Texarkana 2000, pet. denied). One without knowledge of the object and purpose of a conspiracy cannot be a co-conspirator; he or she cannot agree, either expressly or tacitly, to the commission of a wrong of which he or she is not aware. Schlumberger Well Surveying Corp. v. Nortex Oil & Gas Corp., 435 S.W.2d 854, 857 (Tex.1968); Pairett v. Gutierrez, 969 S.W.2d 512, 516 (Tex.App.-Austin 1998, pet. denied).
However, conspiracy may be (and often is) established by circumstantial evidence. Int'l Bankers Life Ins. Co. v. Holloway, 368 S.W.2d 567, 581 (Tex.1963); Lesikar, 33 S.W.3d at 302. The Texas Supreme Court has stated,
A conspiracy may be proven as well by the acts of the conspirators, as by anything they may say, touching what they intended to do.
The general rule is that conspiracy liability is sufficiently established by proof showing concert of action or other facts and circumstances from which the natural inference arises that the unlawful, overt acts were committed in furtherance of common design, intention, or purpose of the alleged conspirators.
....
....It is not required that each and every act of a conspirator be shown to have been in concert with the others or that it be established by direct evidence that all combined at a given time prior to each transaction. Inferences of concerted action may be drawn from joint participation in the transactions and from enjoyment of the fruits of the transactions on the part of the ... defendants.
Holloway, 368 S.W.2d at 581-82; Lesikar, 33 S.W.3d at 302.
The conspiracy issue is common to all plaintiffs. The differences in the sales contracts are relatively minor, and the claims raised against the Dealers are substantially identical.
The Dealers take the position that even if the issues are sufficiently common and predominate, the defenses raised are not. The Dealers argue that in each separate sale, there may have been additional oral discussions that could figure into determining whether a particular customer was informed about the tax, or that some sales could have been negotiated for a final figure, into which the tax was added by the dealerwhile others were cash prices for the vehicle itself, to which the tax was thereafter added.
*404 In support of this position, the Dealers rely on Carney, 37 S.W.3d at 38. In that case, the court reversed a certification because whether the individual class members had actual knowledge of the purported injury was going to be an integral part of the defendant's defense. The holding in Carney rested on the lawsuit being based on the individual employees' knowledge of how their pay schedule was contrived.
In this case, the harm is based on allegations that the Dealers had shown the dealers inventory tax as a tax, which was therefore payable by each customer to a taxing authority. Taxes payable to the government are inherently different from salaries payable by a company. The citizens of this country understand that taxes are not negotiable, that they must be paid, and that refusing to pay taxes results in extremely serious consequences. The question here is whether the documents presented to each customer indicated that the customer owed tax on the purchase of his or her automobile.
In addition, the discovery timely provided to the trial court indicates the tax was mentioned or discussed in only a few situations.
In every case in this class action, the buyer was confronted with an inaccurately described line-item tax that was usually sandwiched in among legitimate taxes payable to the state by the buyer. Further, discovery shows that ninety-eight percent of the Dealers reported that in all but a few transactions, there was no discussion of the tax charge, and it is unlikely that oral representations would be admissible to add to or detract from the written contract in any event.
Under proper application of these considerations, we find the trial court did not abuse its discretion in determining that common issues existed in the conspiracy claim or in concluding that those issues predominated over individual issues.
The only remaining major issue involves the Dealers' contention that the DTPA claim cannot be tried as a class action because there are issues of reliance involved that must be individually proven in separate trials. The two appeals presently before this court involve different versions of the Act. In this appeal, the post-1995 version of the Act is involved. The statute reads as follows:
(a) A consumer may maintain an action where any of the following constitute a producing cause of economic damages or damages for mental anguish:
(1) the use or employment by any person of a false, misleading, or deceptive act or practice that is:
(A) specifically enumerated in a subdivision of Subsection (b) of Section 17.46 of this subchapter; and
(B) relied on by a consumer to the consumer's detriment,....
Tex. Bus. & Com.Code Ann. § 17.50(a) (Vernon 2002) (emphasis added). This version of the statute thus requires the plaintiff to prove that the plaintiff relied on the misleading act. Dealers argue that the class representative is therefore in the position of being required to prove in countless mini-trials that each class member "relied" on the representations of the Dealers before the class member can recover. Therefore, they argue, this individual issue is of such importance that the common issues cannot predominate over it.
Their argument, however, itself focuses on the notion that reliance must be individually proven before a jury can conclude it is the only (or at least the main) reason for acceding to the misrepresentation. The argument is a different flavor of the reliance claim and is based on Peltier Enter., Inc. v. Hilton, 51 S.W.3d 616 (Tex.App.-Tyler *405 2000, pet. denied). The Peltier case concerned the dealers' failure to disclose to consumers they were receiving a kickback (or participation fee) for steering consumers to a particular loan. The result was that the consumer paid a higher interest rate than the bank charged, with the dealer taking the percentage between the bank's charge and the amount paid by the consumer.
In connection with a DTPA Section 17.46(b)(23) (presently renumbered at (b)(24))[6] action, the opinion reviewed certification. That section is not the section in question here and contains different and stronger language. It requires proof of a failure to disclose information, intended to induce a consumer into a transaction "into which the consumer would not have entered had the information been disclosed." That requirement is not part of the statute and need not be met for this case. This variation of a "but for" requirement does not exist under the portion of the DTPA at issue in this case.
That opinion also discussed Section 17.45(5),[7] which also requires a showing of what the consumer could have or would have done if he had known the information. That requirement also does not exist in this case. Peltier is not controlling.
The Dealers also take the position that at the least each class member must demonstrate he or she relied on the misrepresentation. The allegations are that the consumers paid a tax they did not owe because they were billed for the tax by the Dealers. That alone is an allegation of reliance. They direct this court to federal opinions involving other statutory forms of recovery that require some type of reliance to justify damages. They also direct this court to two Texas cases that only allowed class certifications to proceed on proof that the misrepresentations were identical in all cases. Those opinions also contained language warning about the danger of disintegration of a class lawsuit if proof of reliance turned out to require myriads of individual trials to show reliance on an individual basis. See Henry Schein, Inc. v. Stromboe, 28 S.W.3d 196 (Tex.App.-Austin 2000, pet. dism'd w.o.j., reh'g of pet. granted); Life Ins. Co. of Southwest v. Brister, 722 S.W.2d 764 (Tex. App.-Fort Worth 1986, no writ).
Dealers focus on language abstracted from Brister that "fraud in which there were material variations in the representations made or in the kinds or degrees of reliance involved are not suitable for class action." Brister, 722 S.W.2d at 774.
In this case, if we take the Brister language as our mantra, there is no indication from the discovery that there are any material variations in the written representations made. We find the Dealers' argument interesting, but also recognize that in any other context, the Dealers would likely prefer that we not create a new rule of law allowing anything outside the written contract to be considered in determining the viability or intent of an unambiguous contract.[8]
*406 The Houston First Court of Appeals recently addressed a similar matter. In that case, a storage company billed its customers for insurance, which the company then did not purchase. The court found the written misrepresentation to justify class certification, while acknowledging that other cases had held that oral misrepresentations have been found not to justify class certification because they necessarily required separate proof. The court found that the charge itself was sufficient for the purpose of certification. Graebel/Houston Movers, Inc. v. Chastain, 26 S.W.3d 24 (Tex.App.-Houston [1st Dist.] 2000, pet. dism'd w.o.j.). The court concluded that the basis of the lawsuit was the defendant's uniform conduct in its billing practices and affirmed the certification.
Sole causation is not required either by statute or case law. It therefore appears that a single producing cause (or reliance) issueor perhaps a very few variations on that themewould be sufficient to fairly cover all of the defendants. Under those circumstances, we conclude the trial court did not abuse its discretion by certifying the class and entering a trial order.
The allegations are common to all of the plaintiffs. The plaintiffs may clearly seek to prove that the written statements are misrepresentations, while the defendants' attempts to prove otherwise amount to, at most, marginally differing defenses. As pointed out in Chastain, Texas courts have held that even if defendants have a defense against claims by some (but not all) class members, the class may still be certified. Chastain, 26 S.W.3d at 30 (citing Sun Coast Res., Inc., 967 S.W.2d at 537; Microsoft Corp. v. Manning, 914 S.W.2d 602, 613 (Tex.App.-Texarkana 1995, writ dism'd)).
The Dealers also argue that evidence about oral statements should be admitted because the parol evidence rule does not apply to DTPA cases, citing Weitzel v. Barnes, 691 S.W.2d 598, 600 (Tex.1985). That case states categorically that where a DTPA action is based on oral misrepresentations, traditional contractual notions did not apply and the parol evidence rule did not prevent their introduction.
This case is based on written misrepresentations, not oral misrepresentations. That difference alone sets this case completely outside the reasoning of Weitzel. It seems entirely appropriate under those circumstances to hold the writer of a contract to the contract which he or she prepared without allowing evidence of extraneous oral statements.
The Trial Plan
The Dealers also argue the trial court abused its discretion by certifying the class without adopting a proper trial plan after the requisite "rigorous analysis." The arguments raised under that contention are the same arguments disposed of above. Because we find that typicality and predominance have been shown, and because the trial court has laid out in broad strokes the way in which the proceedings would continue, we find the trial plan is adequate for this stage of the proceeding.
Conclusion
Our review is not a de novo review of the evidence; it is an examination of whether the trial court abused its discretion in certifying the class based on the evidence and pleadings. We are required to be certain the trial court did not simply "certify first" without engaging in a serious analysis of the action and the apparent propriety of certifying the class. We are further aware the trial court may modify the class definition or otherwise alter its ruling at any time during the proceeding should it become apparent such a change is necessary.
*407 As set out above, it appears that the trial court could reasonably come to the conclusion that common issues predominate in this lawsuit and that the case can be most expeditiously tried as a class action. We further note the amount of money involved is set out on each separate sales sheet, and may be easily and quickly ascertained and applied, as may be an arbitration provision, should any be contained within an individually written contract. Those are easily determined clerical matters and will not prevent certification.
The trial court did not abuse its discretion by certifying the class. The order of certification is affirmed.
NOTES
[1]  Tex.R.App. P. 28.1; Tex. Civ. Prac. & Rem.Code Ann. § 51.014 (Vernon Supp.2002).
[2]  All class actions must satisfy the following four threshold requirements: (1) the class is so numerous that joinder of all members is impracticable, (2) there are questions of law or fact common to the class, (3) the claims or defenses of the representative parties are typical of the claims or defenses of the class, and (4) the representative parties will fairly and adequately protect the interests of the class. Tex.R. Civ. P. 42(a); see Southwestern Ref. Co. v. Bernal, 22 S.W.3d 425, 433 (Tex.2000).
[3]  Certification is not irreversible, and the trial court may alter, amend, or withdraw class certification at any time before final judgment. Tex.R. Civ. P. 42(c)(1); see W. Teleservices, Inc. v. Carney, 37 S.W.3d 36, 40 (Tex. App.-San Antonio, 2000, no pet.); Morgan v. Deere Credit, Inc., 889 S.W.2d 360, 365 (Tex. App.-Houston [14th Dist.] 1994, no writ).
[4]  In relevant part, the rule reads as follows:

RULE 42 CLASS ACTIONS
(a) Prerequisites to a Class Action. One or more members of a class may sue or be sued as representative parties on behalf of all only if (1) the class is so numerous that joinder of all members is impracticable, (2) there are questions of law, or fact common to the class, (3) the claims or defenses of the representative parties are typical of the claims or defenses of the class, and (4) the representative parties will fairly and adequately protect the interests of the class.
....
(b) Class Actions Maintainable. An action may be maintained as a class action if the prerequisites of subdivision (a) are satisfied, and in addition:
....
(4) the court finds that the questions of law or fact common to the members of the class predominate over any questions affecting only individual members, and that a class action is superior to other available methods for the fair and efficient adjudication of the controversy. The matters pertinent to the findings include: (A) the interest of members of the class in individually controlling the prosecution or defense of separate actions; (B) the extent and nature of any litigation concerning the controversy already commenced by or against members of the class; (C) the desirability or undesirability of concentrating the litigation of the claims in the particular forum; (D) the difficulties likely to be encountered in the management of a class action.
[5]  Noting that courts, before its decision in Bernal, were certifying class actions without conducting the proper analysis into each requirement of Rule 42, notably section (b)(4) of the rule, the Texas Supreme Court expressly rejected an "approach of certify now and worry later." Bernal, 22 S.W.3d at 435.
[6]  Act of May 30, 1993, 73rd Leg., R.S., ch. 570, § 6, 1993 Tex. Gen. Laws 2125, amended and renumbered by Act of May 25, 2002, 77th Leg., R.S., ch. 962, § 1, 2001 Tex. Gen. Laws 1929, 1931, Act of May 27, 2001, 77th Leg., R.S., ch. 1229, § 27, 2001 Tex. Gen. Laws 2822, 2823.
[7]  Tex. Bus. & Com.Code Ann. § 17.45(5) (Vernon 2002).
[8]  Texas courts have held that the existence of differing affirmative defenses will not prevent class certification. Graebel/Houston Movers, Inc. v. Chastain, 26 S.W.3d 24, 30 (Tex.App.-Houston [1st Dist.] 2000, pet. dism'd w.o.j.); see Sun Coast Res., Inc. v. Cooper, 967 S.W.2d 525, 534 (Tex.App.-Houston [1st Dist.] 1998, pet. dism'd w.o.j.); Microsoft Corp. v. Manning, 914 S.W.2d 602, 613 (Tex.App.-Texarkana 1995, writ dism'd).
