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[Diver]

I presently have OEM runflats on my car, 215/40 x18 front and 245/35 x18 rear. These are the same diameter. I would like to replace the front with a 225/40 x 18 and the rear with the stock size which would give the front tire a slightly larger diameter than the rear. My concern is the different speed between front and rear tires would trigger ABS or traction control. I have heard of this happening on other brands of vehicle.

Has anyone run this setup successfully and is there some trick to making it work?

[bmw1racer]

I don't think the change in tire sizes would alter how the ABS or traction control works.

I'm pretty sure those systems monitor the rotational speed of the front tires independent of the rear tires... The only way those systems would get weirded out would be if you had two fronts (or two rears) that were different sizes.

[128Convertibleguy]

Originally Posted by Diver

I presently have OEM runflats on my car, 215/40 x18 front and 245/35 x18 rear. These are the same diameter. I would like to replace the front with a 225/40 x 18 and the rear with the stock size which would give the front tire a slightly larger diameter than the rear. My concern is the different speed between front and rear tires would trigger ABS or traction control. I have heard of this happening on other brands of vehicle.

Has anyone run this setup successfully and is there some trick to making it work?

WARNING I have no professional expertise in this area. Here are some clues.

I've seen a 3% difference suggested as the lower limit for any problems. Yours would be 1-1/2 %, if the tires were the same tread depth.

And, if your present tires were full tread depth on one end, and about half tread depth on the other, you'd have about the same difference. It doesn't seem likely the system would fail in that situation.

You could get over 3% difference if the fronts were full tread depth and the rears more than half worn.

An interesting take away is that speedo error is slightly changed by worn tires at the end which drives the speedo. From new to 2/32 left would change the diameter about 2%. That's about 1 MPH at 60.

Yeah, that's clear as mud.

[Diver]

Originally Posted by 128Convertibleguy

WARNING I have no professional expertise in this area. Here are some clues.

I've seen a 3% difference suggested as the lower limit for any problems. Yours would be 1-1/2 %, if the tires were the same tread depth.

Yeah, that's clear as mud.

I searched through this forum and found 2% suggested as the limit. Following this line of thought, the starting difference would be 1.3% (per Tire Rack specs). However, the front is larger and the rear wears about twice as fast. As the tires wear, passing 2% seems very likely, and 3% possible.

It is starting to look like changing the size of the front only, is not such a great idea.

[Olgeezer1]

Don't most of the 135 guys use 225 front and 255 rear? As you say, don't think changing front size only is the best idea. I kept my stock staggered wheels(same size as yours) but squared up w/225/40/18 all around. Works great for my 128, much better balanced, but probably not enough rear tire for a 135.

[Diver]

Originally Posted by Olgeezer1

Don't most of the 135 guys use 225 front and 255 rear? As you say, don't think changing front size only is the best idea. I kept my stock staggered wheels(same size as yours) but squared up w/225/40/18 all around. Works great for my 128, much better balanced, but probably not enough rear tire for a 135.

Going to a larger front tire is a quick fix for under steer, but I think the 225/255 setup is popular not only because it avoids the tire size problem, but because there is a far better selection of tires in those sizes.

At this point I feel there are two options for me, go with stock size PSS all around, or just replace the rears with run flats. The fronts are in great shape.

[Pyrat 2]

Running 235/40 front and 245/35 rear MPSS with no DSC or ABS issues. I will be switching to 245/35 square when these tires are spent just to get square.

[Diver]

Originally Posted by HeelToe

Running 235/40 front and 245/35 rear MPSS with no DSC or ABS issues. I will be switching to 245/35 square when these tires are spent just to get square.

That would be a 2.4% difference with the larger front tires if all tires were new. Are your rear tires worn noticeably more than the front?

[Pyrat 2]

Put them on as a set. I'm actually on my second set of fronts for the same set of backs. Sounds unusual but I toasted a set of fronts at the track before I added some additional camber. Now all is well. The fronts still get a little extra wear on the outer edge than the rears. Other than that there is no appreciable difference in depth.

[Diver]

My car is not tracked so the rears wear much faster than the front tires. I have heard of other members of this forum trashing their front tires pretty fast at the track.

[Gary@TireRack]

225/40R18 up front with 245/35R18 rear should cause no problems with ABS. We've seen countless people run this setup for years with no problems.

Tires http://www.tirerack.com/a.jsp?a=AW7&...ires/index.jsp

[Diver]

Originally Posted by Gary@TireRack

225/40R18 up front with 245/35R18 rear should cause no problems with ABS. We've seen countless people run this setup for years with no problems.

Tires http://www.tirerack.com/a.jsp?a=AW7&...ires/index.jsp

Thank you Gary.

[Diver]

I finally pulled the trigger on a set of PSS and I am very happy. Harshness over bumps is gone, less road noise, no float and the traction control light isn't coming on all the damn time. In all fairness I had worn out rear run flats that did not stick very well, but I am not seeing that light in normal driving, and even before the run flats were shot it was a regular reminder. This car does not get tracked, but I went around some ramp today at a speed that I can't publish here and the car was sticking nicely.