Can someone please tell me the whole thing about the overall tire diameter difference thing with the wheels and the ABS/TCS system? I'm trying to run a 18/19 setup. The thing I'm confused about is, is this regulated by the stock tire overall diameter? Is ok to have bigger tire diameters than factory just as long as the front and rear are close. I really want to run 275/35/19 in the back and 255/40/18 in the front. I've searched all over and found mixed results. Some saying they haven't had issues and others with problems. I don't wanna go out and buy a grand worth of tires just to find out I wasted my money. PLEASE! Somebody has to have some answers.

 

ABS works by detecting differences in the rotation speed of the hubs. There's lots of math and hocus pocus that goes on in the ABS module to figure out which readings denote a slip. Once a slip is detected, the ABS goes in to action.

Or, if the ABS module sees unnatural results from the corners, it could throw a code instead.

The key measurement is the tire diameter. (I've attached a simple spreadsheet that calculates this measurement.) Since the tires you want are a larger diameter than the stocks, the wheel hubs will rotate slower for any given speed. (This also throws off your speedometer.)

For example, if your rear wheels are larger and your front wheels are stock and you are driving in a straight line and you are under acceleration: your ABS module will see your front wheels moving "faster' relative to the the rears. (Like a front wheel car doing a burnout.) Since you aren't in a front wheel drive car, I imagine this might throw a code. If you are under braking, the ABS might interpret this to mean your rear wheels are slipping/skidding, causing the ABS to be activated, etc.

So, if you get a tire combination that is closer to the stock diameter, it should cause fewer problems. Otherwise you may need to intervene with the ABS system. (Not sure if new tire sizes can be programmed in, or not. I expect most people who change their tire sizes for racing would do an ABS delete.)

 

I'm still a little confused. I understand that you want your front and rear to be as close as possible. But, why does your tires need to be as close to original as possible. For instance, if factory is 25.2 and you choose to put front and rears that are both 26.5. How does this set the abs off. To me as long as the fronts and rear are turning the same speed it shouldn't matter right? The difference in tire size between the front and back of the tires I want to run is about 2%. But if I compare them to factory overall diameter the front difference is 1.5% and rear is 3.51%

 

You are correct in that it is the difference in the front to rear split that counts. Some vehicles I've worked with run into problems at difference more than 3% of the original front:rear split. The most sensitive was the Mercedes McLaren SLR. I'm not exactly sure where the 4th Gen lies, but 2% should be fine.

Chris

 

Thanks this the answer inwas looking for. I just didn't understand how all these 4x4 guys could run giant tires and we cant run a tire that's a inch bigger than stock. So as long as I keep the front close to rear I think I'll be good. Thanks.

 

Right. Same diameters front/back will keep your ABS happy and the stock diameter will keep the spedo true. (But that can be fixed in the tranny or with a tune.)

I like the look of a staggered front/back and hope to figure out a way to tune the car to be happy with the setup.