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  1. #1

    Question Effects of running equal diameter front and rear tires

    I've searched and not been able to determine for sure the issue with running equal front and rear tire diameters. On the previous generations there was no stability control to worry about but the car would develop a serious and arguably dangerous rear brake bias. Does anyone have first hand experience running equal or near equal diameters (within 0.5")? Does the computer freak out?

    Thanks

  2. #2
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    This will 100% cause issues. The rear tires are much taller than the front, you've got to keep it that way. A little change in height is ok, but not several inches.
    Here's the stock heights:
    Base cars
    295-30-18 25"
    355-30-19 27.4"

    ACR
    295-25-19 24.8"
    355-30-19 27.4"

    We are talking 2.5" LOL.
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  3. #3
    Quote Originally Posted by Arizona Vipers View Post
    This will 100% cause issues. The rear tires are much taller than the front, you've got to keep it that way. A little change in height is ok, but not several inches.
    Here's the stock heights:
    Base cars
    295-30-18 25"
    355-30-19 27.4"

    ACR
    295-25-19 24.8"
    355-30-19 27.4"

    We are talking 2.5" LOL.
    Thanks for the reply,

    I am aware of the OEM diameter delta. You sound very sure. What issues will this cause? Do you have first hand experience?

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    I've heard of some having issues with the computer for stability and abs while others never having an issue.

    No first hand experience yet but I plan on trying out R888R either 295/30/19 or 305/30/19 which are 26" or 26.3". Rear would be 345/30/19 at 27.2" tall.
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  5. #5
    Yes it will cause issues, bucking, jerking, throttle surge, all kind of weird things.

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    Quote Originally Posted by Scot@Prefix View Post
    Yes it will cause issues, bucking, jerking, throttle surge, all kind of weird things.
    Sounds like a no-go!
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  7. #7
    Quote Originally Posted by Scot@Prefix View Post
    Yes it will cause issues, bucking, jerking, throttle surge, all kind of weird things.
    Thanks. To confirm, you experienced these things? Which tire sizes were you running? This was with the Stability Control turned off or with it on?

    Thanks

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    Quote Originally Posted by Sprockett88 View Post
    Thanks. To confirm, you experienced these things? Which tire sizes were you running? This was with the Stability Control turned off or with it on? Thanks
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    I tried to run 295-30-19's on the front of my ACR-E. It caused it to go into a limp mode as soon as I got into second gear. The only way I could run this size tire was to put it into full off mode for traction and stability. It would take a couple of laps for the computer to get happy with this. The anti lock braking system would still act like you were at anti lock braking even though you were hardly braking. Scary when coming up to a turn and it won't slow down. Talked with a lot of other ACR-E owners and they could run the 30-19's with out this issue but any bigger and they didn't work. (Limp mode)
    Was trying to run slicks without buying new rims. Ended up getting some 18" forge lines. Specially made to fit over the calipers.

  10. #10
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    Any suggestions for 19" tires. The 295/30 PS4S tires are 26", rubs alot.
    Thinking of going down to a 285 width which makes the tire height 25.8". Need to look up what 275 will do, but 275 and 345, front and rear, is that too much of a difference?
    Last edited by serpent; 09-13-2019 at 11:29 PM.

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    Quote Originally Posted by Sprockett88 View Post
    Thanks. To confirm, you experienced these things? Which tire sizes were you running? This was with the Stability Control turned off or with it on?

    Thanks
    2016 ACR-E - I tried 335/30 front and 345/35 rear Hoosier A7s on 18 inch Finspeed wheels. With all traction modes on, it went into limp mode in the first corner after going on track. It cleared down the straight and I came in. The next attempt was in track mode. One okay warm-up lap, then in the first hard, high-speed sweeper it went to limp mode (idle) and would not clear until I shut the motor off and restarted. Maybe if I had driven a few laps it would have "learned", but I wasn't in the mood - too hot and too much traffic. I am going to try again when the weather cools down a little. Several others have run the same tires and after a little time, the computer seems to accept the condition. We'll see.

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  12. #12
    Quote Originally Posted by Sprockett88 View Post
    Thanks. To confirm, you experienced these things? Which tire sizes were you running? This was with the Stability Control turned off or with it on?

    Thanks
    One of our customers, experienced this on a gen 5 GTS, I dont remember the size off the top of my head.
    Thought he had an engine issue lol
    Ran the sizes by Arizona Viper and he confirmed the sizes were too far off for the computer to think right lol

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    On my 2016 ACR (non E) w/ceramic brakes I've run:

    Front - Forgeline GTD1 19x11.0 + 30 with Toyo R888 295/30-19
    Rear - Forgeline GTD1 19x13.0 + 62 with Toyo R888 345/30-19

    I've had no issues with all "help" turned on at COTA. Maybe I'm not running as hard as the rest.
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    Quote Originally Posted by Scot@Prefix View Post
    One of our customers, experienced this on a gen 5 GTS, I dont remember the size off the top of my head.
    Thought he had an engine issue lol
    Ran the sizes by Arizona Viper and he confirmed the sizes were too far off for the computer to think right lol
    Scot - actually these were a set of tires Arizona Viper ran a couple of laps on at COTA in his ACR. He didn't have trouble with them, but I think I recall him saying he had a little trouble when he first started running that size, but it cleared up. I will go to a test-and-tune track day and run a few more laps to see if it gets better.

    Pappy
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    My experience on the TA is they must be at least 1” difference. This setup works with Toyo RR’s, but had issues with R7’s. Both 295/30/19 and 345/30/19. Online sizes are 26.1 and 26.8 for r7’s. 26 and 27.2 for RR’s
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    Last edited by Ogme042; 09-14-2019 at 04:52 PM.
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    Both traction and ABS determine wheel slip based on preset parameters loaded into the system. These parameters are based on OEM tire sizes. If you change tire sizes too much the ABS and traction systems believe an axle is slipping relative to the other because one axle is turning too fast based on OEM tire sizes. This can cause the traction control system to operate when it shouldn't and it can cause ABS to operate improperly.

    So let's assume you change rear tire height to be smaller, maybe similar to fronts. Because the diameter is smaller the tire must turn faster to go the same MPH. The traction control system looks at the speeds of both axles and says "the rear axle must be slipping because it is turning faster than the front at this MPH." It then intervenes by activating traction control on rear axle. Similar to what happens if you light up the rears with the throttle. The rears start turning faster than would be indicated based on front axle speed. Ditto with ABS, the system will assume that because one axle is spinning faster than it should the system thinks that one axle is slipping relative to the other under braking and it then activates the ABS.
    Last edited by ViperGeorge; 09-14-2019 at 05:38 PM.
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  17. #17
    Quote Originally Posted by Pappy View Post
    Scot - actually these were a set of tires Arizona Viper ran a couple of laps on at COTA in his ACR. He didn't have trouble with them, but I think I recall him saying he had a little trouble when he first started running that size, but it cleared up. I will go to a test-and-tune track day and run a few more laps to see if it gets better.

    Pappy
    The customer i am referring to was not Arizona Viper. It was a different customer

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    Quote Originally Posted by Ogme042 View Post
    My experience on the TA is they must be at least 1” difference. This setup works with Toyo RR’s, but had issues with R7’s. Both 295/30/19 and 345/30/19. Online sizes are 26.1 and 26.8 for r7’s. 26 and 27.2 for RR’s
    Ah, I think you may have cracked the code. Could explain why some have issues and others do not. I will go with 295/30/19 instead of 305 based on that with the Toyos. Any rubbing issues up front? Nice TA by the way!
    2016 Viper TA ~ ARH Headers/Exhaust/Arrow PCM ~ 11.1@129

  19. #19
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    Based only on doing the math from the sizes of tires and not with a tape measure (and assuming the widths are exact to an infinite number of significant figures...)...

    OEM Front: 295/30-18 = 24.9685 inches tall.
    OEM Rear: 355/30-19 = 27.38583 inches tall.

    I've been running the following sizes on my '14 with no problems aside from the occasional ABS/ESP wiggle for the first couple miles after swapping between different sets of wheels:

    Front: 305/25-20 = 26.00394 inches tall.
    Rear: 345/25-20 = 26.79134 inches tall.

    That's less than an inch difference front to rear, so... inconclusive.
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  20. #20
    Sounds like the best course of action is to stick with the stock recommended sizes and avoid computer headaches. With the Gen5 cars handling so well, I'm not certain if there would be any noticeable traction gain by changing. Maybe someone who has done so will comment based upon their experience.
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  21. #21
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    Quote Originally Posted by Pappy View Post
    2016 ACR-E - I tried 335/30 front and 345/35 rear Hoosier A7s on 18 inch Finspeed wheels. With all traction modes on, it went into limp mode in the first corner after going on track. It cleared down the straight and I came in. The next attempt was in track mode. One okay warm-up lap, then in the first hard, high-speed sweeper it went to limp mode (idle) and would not clear until I shut the motor off and restarted. Maybe if I had driven a few laps it would have "learned", but I wasn't in the mood - too hot and too much traffic. I am going to try again when the weather cools down a little. Several others have run the same tires and after a little time, the computer seems to accept the condition. We'll see.

    Pappy
    I wonder why you're having issues - I know I'm not the only one running 315/30-18 & 345/30-19 [25.44"//27.15"] without issue; your difference is still closer to stock [25.44"//27.51"], unless they made computer changes in later years or acr specific I don't think the calculations would change for it with regards to abs/traction intervention.
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  23. #23
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    Quote Originally Posted by dirtyboot View Post
    I wonder why you're having issues - I know I'm not the only one running 315/30-18 & 345/30-19 [25.44"//27.15"] without issue; your difference is still closer to stock [25.44"//27.51"], unless they made computer changes in later years or acr specific I don't think the calculations would change for it with regards to abs/traction intervention.
    Actually, the 335/30X18 A7s (front) are 25.6 inches and the 345/35X18 (rear) are 26.8 inches. I think that is in the range of what the computer can "learn" after a few laps. It has worked for others. The Hoozier A7 345/30X19 is also 26.8 inches, so no help. As far as rationale for switching tires, there are 2 reasons. The A7s are faster, and I'm not fond of going through front Kumhos in a single track weekend (6-8 sessions). I personally am not looking for aesthetics, and most of the good track tire selection is in the 18" size. If you are not looking for real track performance I can see sticking with the stock sizes/tires.

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    Quote Originally Posted by Sprockett88 View Post
    Thanks for the reply,

    I am aware of the OEM diameter delta. You sound very sure. What issues will this cause? Do you have first hand experience?
    If you have the same diameters, hence the same height front and rear, you're eliminating the rake completely which is really bad and the car would handle like crap.
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