ABS and Tire Size

[DaleG]

How does tire size (dia.) affect ABS performance? Do front & rear tires need to be ~ same diameter?

[CraigS]

Absolutely they need to be the same. My experience is that you can get away w/ full tread (usually 10-11/32) on one end and 2-3/32 tread depth on the other, but that is about it. The ecu measures wheel speed and it is very accurate. The allowable variation isn't due to insufficient ability to read small variations. It's due to being programmed into the system. The systems go through an initial check after every start within the first fifty yards. At that point it's just looking for a signal, any signal. From then on it is looking at what the signal actually is telling it about wheel speed etc. One thing to note, (and this applies to daily drivers too) if you have an ABS warning light, the system is shut down. Once it kicks that light, it is no longer operating. Same goes for airbag or SRS systems.

[Jeff Kleiner]

I've heard a 1% maximum variation front to rear, which happens to come pretty close to Craig's numbers if you do the math for a 25" diameter tire.

Jeff

[DaleG]

So, my 0.6" difference between front/rear tire diameters is too much (26.1" vs. 26.7"); 1% = .26".

Thanks. More checking, I guess.