Looking to put a sticky tire on the rear of my procharged Ram air.

Have CCW 18’s on the car now.Front tire size - 275/35/18
Rear tire size - 315/30/18

I want to put 315/35/18 Toyo TQ’s on the rear of the instead of the 315/30/18 BFG Rivals on the car now. Is the difference in diameter (1”) too much for the ABS?

Will the abs cause problems at 1” or just a bit more difference?

OD - 25.55” (front) vs 26.60” (rear) so just over 1” difference

Thanks!

 

I’d say you should be okay, but it’ll be close. I think 4% I read was the tolerance for height difference

 

Yeah, I think anything over ~1" difference (for heights near the stock 25.66" range) is right near the tipping point from what I remember. It will be close.

 

Thanks yall, ordered the tires, and they will be delivered Wednesday. I'll update as to how the ABS reacts (or doesn't)

 

Welp Damn....

Installed the tires, and abs love the traction they provide.

Unfortunately, they are throwing the ABS for a total loop and making it engage at higher speeds when normally it wouldn't.

What are my options now? Will a tune defeat this (adjusting the tire size), or will I need to install a taller front tire instead?

Thanks to all for the input!

 

I don't think this can be fixed by adjusting tire diameter in the tune, I think it's a direct matter of what the wheel speed sensors are seeing. Not sure if there is a hack/fix for this, other than a closer match of tire height, but maybe someone has found an alternative.

 

So I've had these same problems for years now and never figured out a fix for the ABS activating at higher speeds, read several threads on here about others having the same problem with no answer or solution to why it happens. All I know is some guys that run taller 28" slicks in back with a 26" front runner or stock tire usually run into problems with the ABS shutting off and tripping the ABS and brake lights on the dash once they get over a certain speed..however mine has never done this..never any lights on the dash, not even when I observe the problem when braking at a higher speed (attempting to slow down at the drag strip after the finish line for example) and I've always run a "26 inch" tire in the front and rear, I put quotes around that because they aren't actually 26 inches..the front nitto 275/40/17 measures 25.67" and the rear mickey thompson 315/35/17 measures 25.91" which is less than 1/4" difference. I'm curious what symptoms yours has..and did it have this problem before switching tire sizes? Also before anyone says its the rotors or pads or air in the lines...I pulled ABS fuses and tested it out and it brakes hard with no rock hard pedal.

 

Are you saying that you're having trouble with a spread of only 0.24"? If so, that's not the same problem that folks usually observe when the height spread reaches ~1" or more. I ran a height spread of 0.32" for many years and never had any issues with ABS lights nor operation at any speed. If you're having these kinds of troubles with such a small spread, it's probably something to do with a wheel speed sensor or some other malfunction. But when it only happens at a larger spread (like ~1" mentioned above), the issue isn't a malfunction but just a side-effect of the larger-than-designed spread.

 

I ran the stock speedlines with 275/40/17s all the way around for a season when I first got the car and it never had any problems from what I recall, wasn't until I put the ZR1s on with a 315 in the rear that I noticed the weird braking, there's times that it acts weird at even lower speeds when attempting to brake hard, pedal gets stiff and just doesn't want to stop well, and is exacerbated at higher speeds, feels scary trying to stop it at the track, have to get off and back on the brakes several times before the pedal feels normal. During normal driving and braking it feels fine, no issues with ABS activating. There's never any lights on the dash to indicate a fault or anything, from what I've read the ABS modules for these cars are discontinued and only available used if you can find one so that's a crap shoot. Not sure where to start as far as diagnosing a problem. In the mean time I have the fuses pulled so it will stop if i have to panic brake.

 

Still trying to fully understand the scope of your problem...

So all the issues you've mentioned, like the hard/abnormal pedal feel, weird braking at high or sometimes even low speeds, having to pump the pedal to get more normal braking feel, etc.... was all of this "corrected" by pulling the ABS fuses? If so, that seems to point to a fault with the ABS module or a wheel speed sensor or wiring/signal problem. If not, then I wonder if there is an issue with the master or booster? I've dealt with booster and master failure (both partial and full) on my '71 and '74, obviously these are not ABS cars but some of the symptoms were similar to what you describe. Booster failure symptoms can be especially intermittent as conditions will change with engine vacuum fluctuation and longer/harder braking duration. But having said that, I don't think I've ever encountered a booster failure on one of these cars - that's usually a problem that develops when a car is much older (due to natural deterioration of the rubber diaphragm).

And you're saying that none of this started until after moving to the 315 tires? You mentioned that your 315s are 0.24" taller but the tire calculator that I use shows a 315/35/17 tire is only 25.68", which is just 0.02" taller than the 25.66" 275/40/17. That is most certainly not enough difference to cause any ABS side-effects by itself. Where did you get the 25.91" measurement from? And did you ever put the old wheels with 275s (squared) back on just for a test to see if anything changed with the problem?

 

the measurement of my specific tire I got from summit's website, the mickey thompson et street R is listed as 25.91"..but I agree, I don't think the tires are the problem, may just be a coincidence that the problem started happening after putting these wheels on. As far as the booster goes it holds vacuum for months and acts normal when pumping the pedal. I still have the old wheels but the tires are 22 years old..feels a little nerve racking to take it up to 70 mph to do a brake test on old dry rotted tires lol. The fact that the pedal feels normal when hard braking with the fuses pulled leads me to believe the master and booster are probably fine. I'm just not sure what to start with for diagnostics short of just replacing ABS parts.

 

this is EXACTLY the symptoms my car has after installing the taller rear tires.

no abs light, no trac light etc, just the pulsating brake, particularly at higher speeds or sudden hard stops. It can be unnerving for sure.

funny thing is a previous setup I had with 17/18 staggered combo and 1” diameter difference did the same thing.

i changed to equal OD tire height and it went away. Now it’s back.

a 275/40/18 puts me right at the same height front / rear. I’m thinking this route to fix the issue. In the meantime, maybe pulling the ABS fuse

 

Yep. This is exactly the side-effect that I would expect from a ~1" mismatch in tire diameter. So that's no surprise and, as you said, the simple fix is to change to a taller front tire to match.

But BFK86's problem is different because he doesn't have a height mismatch that's significant enough to cause this problem.

I think Summit made an error in that height info, or perhaps MT provided incorrect info. I could see different brands being off by a small margin of error, but 1/4" height difference seems pretty excessive at the same size. 315/35/17 should be 25.68". So that is definitely not your problem IMO. Anyway, as I mentioned earlier I was running a 285/40/17 rear tire for many years, which is 25.98", vs. the 275/40/17 (25.66") front, and I didn't have a single problem doing this, even braking at speeds of 100+mph.

Here is the tire calculator I use for reference:

https://www.discounttire.com/learn/tire-size-calculator

So yeah, I agree that it's not the tire height and that something is definitely malfunctioning in your case, but I'm not really sure where to start with diagnostics either (especially since you're not getting any sort of dash lights). I wonder if a more comprehensive scanner that can read brake system info would show something even if you're not seeing an ABS INOP, etc.?

 

Yeah I was considering buying a scanner that has the ability to "trigger" certain things like the ABS pump to attempt to bleed the brakes while cycling the pump. I know nothing about scanners though so I have to figure out which one to buy that isn't super expensive but can also do some of those functions.

 

When bulling the ABS fuse , there are numerous ones in the boxes under the hood, is there a specific 1 to pull? I am also runing staggered tires and see many threads regarding pulling th efuse, but not sure which one to de-activate teh sustem? Thanks!