Hi, does anyone know what the max front to rear tire size difference is before the ABS has issues. For the GT2. I read lots of posts but they were all about the Turbo which is AWD etc not rear driven.

Factory 235/40/18 to 315/30/18 is 0.15% difference.

I am keen to know if rear 295/35/18 (2.64% difference) or 295/30/18 (-1.73% difference) will work. I am wanting to try a new semi slick and that is as big as they make the rears. I had heard 3% is the limit before ABS has a dummy spit?

I appreciate any guidance. Thanks.

 

yeah. it's less than the oft mentioned 3%. the psm triggers sometimes merely with new rears and old fronts. the rolling diameters are not to be messed with. below are the numbers and optimal sizes for stock vs gt2. this may or may not help, that's all i've got.

Spec Sidewall Radius Diameter Circum Revs/Mile Difference
225/40-18 3.5in 12.5in 25.1in 78.8in 804 0.0%
235/40-18 3.7in 12.7in 25.4in 79.8in 794 1.3%
295/30-18 3.5in 12.5in 25.0in 78.4in 808 0.0%
315/30-18 3.7in 12.7in 25.4in 79.9in 793 1.9%

 

PSM wasn't available on the GT2. Without AWD, does tire diameter difference matter?

https://www.6speedonline.com/forums/996-turbo-gt2/271869-tires-size.html

 

an excellent and salient point. i can't see how it would, but now out of my depth. i will now have a coffee.. or two

 

The size difference does not matter on the GT2 since you do not have PSM. ABS only comes into play when it senses that a wheel is about to lock up. Nothing more, nothing less. My car is RWD without PSM, only ABS, and have run various combinations with up to 1" difference in size front to rear without any issues whatsoever.
Have fun....

 

That's interesting. I always thought the abs was using some form of speed differential that would include front to rear measurement, does anyone know more?

 

That is the function of PSM as far as I know.

 

GT2 doesnt have PSM.

It does have ABS. Hence my questions.

 

I was running 295/35/18 and 235/40/18 for over 15000 miles on my AWD turbo with out any problems.
When I measured them on my rims, there was a . 5 '' difference.
You shouldn't be concerned because you are rear wheel drive only..

 

I'm converting my 03 GT2 to full road race and am running a 28" rear with a 25" front (997 RSR sizes 27/65 and 31/71) which is a huge difference in diameter. While there is no PSM, I have been told the ABS will have to be recalibrated which is done with a new internal pump gear. I was told it's no big deal but must be done or the (stock) ABS will indeed freak out. The ABS does constantly compare wheel speeds and kicks in when there is a programmed difference. Logically, you would think tire diamter is rather critical in this calculation...I plan on trying the stock ABS out and if it does not work convert to Tevis which is a far superior system anyway.

 

What you can do is work the numbers -- circumference -- of the officially sanctioned wheel/tire combos and come up with some new front tire to new rear tire circumference differences that are acceptable.

Then work the circumferences assuming the left and right tires differ by 30% in their tread depth. This 30% comes from Porsche in that it says when replacing a tire on an axle both tires should be replaced when the tread difference is 30% or more.

Doing the above will give you a good idea of what front and rear tire circumferences work with no problems.

Then do the circumferences for the tire/wheel combinations you want to run. If the circumferences fall into the safe range, fine.

If not, not fine.