cliffyk

Learn to live with it, or put the same size tires on the front too--AFAIK there are no provisions in the PCM tune to accommodate different tire sizes on the front and rear axles.

Those are big-*** tires, the stock tires are 25.7" with no load on them, 24.9" or so working diameter when loaded. It's a 8% increase, just a bit less than going from 4.10s to 3.73s (+10%)...

devongarver

Funny you mention the gear ratio concept cliff...couldn't he actually get around it that way, by programming for a different gear ration out back that matches his tire size? Then it would work correctly no? Oh wait...nvm no it wouldn't the ABS sensor isn't linked in any way to the speedometer...FML

cliffyk

There's also a "Tire Revs per Mile" scalar in the tune that is intended for correcting the speedometer. The standard value for 245/45-17s in 812 revolution per mile (24.85" working diameter).

For the OP's 27.7" diameter tires the value would be 736 or so.

However as you stated this will only fix the speedometer readout, it will not correct for the ABS' thinking that the wheels are spinning because of the difference in front to rear tire sizes--which I have assumed is the case?

devongarver

Exactly, if I remember right, I think I read that about 3% height difference is all you can go before having ABS issues. I may be wrong though.

cliffyk

Yes, now that you mention it I have read that as well.

3% is a good sized difference, at 60 with the stock tires that would mean there was a 24 rpm difference in tire speeds.

devongarver

Now here is a good question, does the abs sensor use a digital or analog setup I wonder? And does it literally count teeth per second(or other time constant) or does it use a "mean" voltage feedback referenced against a preset table to calculate the wheel rpms? Do you know what I am trying to say? The faster the wheel spins, the more often there is a tooth present per given time constant, which could be seen by measuring the average inductance per time constant, and as it would have a direct relationship correct?

Maybe I'm just a tard lol

EDIT:

If I knew how this worked, possibly a module could be made to plug in to the abs harness in between it and the abs sensor, to "accommodate" for different size rear tires and keep the abs function intact!?

cliffyk

The sensor is a Hall-effect device, it produces an output pulse each time a cog of the rotor passes close by. The ABS module uses Schmitt trigger input to clean up the pulses and then counts them (versus time) to determine each wheel's rotational speed. The rest is just firmware and of course the output circuits to control the ABS pump and valving.

Not a bit of rocket science, you could do the same thing with a 50¢ 8-bit PIC and a handful of components...

devongarver

Man,....I REALLY need to go back and get refresher courses...this is all common sense junk if I could remember half of what I learned in school....I get dumber and dumber as the years go by, and Im only 24...I'll be put in a home before I hit 40 at this rate lol

2kStangGT08

Do they make a abs delete manifold?