I have ABS on my car (fortunately or unfortunately, depending on how you look at it) and I'm wondering if putting taller tires in the rear is going to make the ABS go nuts.

Front would be stock sized SE tires (215/60/15 or 25.2" tall) and rear would be taller and slightly narrower (205/70/15 or 26.3" tall).

 

The difference is about 4%. Normal variation should be about 1-2%. So the ABS may think the rear tires are locking up a little sooner than reality. Then you may notice ABS activated a little more frequently than normal.

 

Thanks. Anyone else had experience with ABS/different diameter tires?

 

The ABS reads wheel RPM. It would never know that your tire is 25" dia or 40" dia.

Dave

 

Similar question (hope you don't mind JClaw!)...does rim size affect it?

 

It does not know absolute tire diameters, but it works by comparing wheel rpm at all four corners which is affected by tire diameter.

I once had a tire shop screw up and mount 205-60-15 snowtires on the rear and 205-65-15 (correct size) on the front. That is approximately a 3.2% difference in diameter.
It made the ABS extremely sensitive. So much so that I pulled the fuse until I had a chance to get back and have them put the correct sized ones on.

JClaw, the 4.3% difference in your planned setup may be too much. The ABS may kick in as soon as you touch the brakes.

 

Rim size does not affect the ABS as long as you adjust tire profile to maintain the same overall wheel diameter.

This calculator is a useful tool for figuring out what will work.

http://www.1010tires.com/TireSizeCalculator.asp

 

Your are right that it reads RPM. The larger diameter tires means larger circumferance. Larger circumferenace means lower RPM for the rear in this case. 4% at 50MPH means the car will detect the rear to be going about 48MPH. The ABS controller compares all for wheels to detect a lock up situation. So for 2 wheels rotating at about the same rate, it may not trigger the ABS pump. But likely it will still trigger a little more than usual.

 

on my Blazer it activates more often with bigger tires so i would assume the same would happen on the max too

 

Normally it wouldn't kick in the ABS pump more frequently if all 4 tires are the same size. So some issue is at play in your case. Some cars are shipped with larger rear tires, so the ABS designers put in more complexed logic to detect a lock up.

 

dont want to hijack the thread.. but on my truck all tires are the same size, rims are american racing rims.. if i put the origional rims and tires on the truck is fine.. but when i have the aftermarket rims and larger bf AT tires on my abs does kick in a little more during cold wet days

 

so then with the space saver spare the abs must be all wacked out.

you may just need to remove the fuse if braking is irratic.
are you planning on driving daily with the miss matched tires?

 

Most standard ABS systems operate on single wheel speed, unless the car has a stability package or traction control. I have run diff wheel diameters on my 300ZX on and off the track and never had an issue with ABS.

 

Our 95 Blazer had the worst ABS...it was so sensitive. I live on a bumpy dirt road and as soon as you hit a washboard section you could hear it buzzing!

 

ABS is likely to be the least of your problems.
I once tried putting larger tyres - not a lot larger - on the rear end of my TR6. (A fast 2.5L Triumph 2 seater sports car from the 70's). The handling went from great, to really, really scary. I've always kept the same size all round since that experience.

 

^^ as true as that may be. i think jclaw is more of a 1/4 mile runner where handling means nothing.