Allowed front tire / rear tire diameter difference

[Alatalo]

What is the consensus on the allowed diameter difference between the front tire and the rear tire, without triggering the ABS light or causing the ABS and / or traction control to interfere during normal driving...?

I did use the search function and I did read through a number of older threads touching the subject, but the information I found is somewhat conflicting. Without quoting individual members, somebody advices to stay inside a 3% rule of thumb. Somebody else says it's more like a 5% rule of thumb. Several members say they get away with a full 2" (~8%) diameter difference, without getting the ABS light or noticing the ABS interfering. However, at least one member says he can notice the ABS stepping in very early already at a 1" (~4%) diameter difference, compared to when he is running identical height front and rear tires.

Is there a consensus on this topic, or is it just a "hit and miss"...? If that's the case, why would different S197's act differently...? Is it down to model year or something else...?

Advice, facts as well as opinions are welcome. The reason for the question is that I am strongly considering a dragrace setup that potentially involves a diameter difference between the front tire and the rear tire.

 

[86GT351]

This is what I use to help customers. https://tiresize.com/calculator/

 

[Alatalo]

Thank you Sir,

Personally, I have just used the calculator on my phone. Or this one: https://www.willtheyfit.com/

May I ask what your experience regarding this particular topic is…? Do you set all of your customers to the same diameter front and rear, or do you know a diameter difference that for sure works without triggering the ABS light or the ABS function itself…?

 

[pass1over]

I currently have 265/35/20 fronts and 305/35/20 rears. This is a 4% difference, I notice nothing wrong with driving my car. Before that, I had 255/35/20 fronts, which is a 5.2% difference. Again, no issues, for 60k+ miles on that set. I've got almost 250k on my car with basically that tire setup the whole time.

 

[86GT351]

Thank you Sir,

Personally, I have just used the calculator on my phone. Or this one: https://www.willtheyfit.com/

May I ask what your experience regarding this particular topic is…? Do you set all of your customers to the same diameter front and rear, or do you know a diameter difference that for sure works without triggering the ABS light or the ABS function itself…?

I/We let the customers make the decision on there own and we document accordingly on the Repair Order. We show them the calculator which clearly shows a difference.

 

[pass1over]

I would get them as close to possible for aesthetic reasons as well. It would look weird for the front sidewall to be taller than the rear. You can get away with a thicker/taller sidewall on the back compared to the front, but too much of a difference would look a little odd on that setup as well. IMO at least.

 

[Alatalo]

I currently have 265/35/20 fronts and 305/35/20 rears. This is a 4% difference, I notice nothing wrong with driving my car. Before that, I had 255/35/20 fronts, which is a 5.2% difference. Again, no issues, for 60k+ miles on that set. I've got almost 250k on my car with basically that tire setup the whole time.

Thank you Sir,

Any chance I can ask you about your exact model, model year, gearbox, brakes and options package, just in case there is something in it that differs between different cars…?

 

[pass1over]

Thank you Sir,

Any chance I can ask you about your exact model, model year, gearbox, brakes and options package, just in case there is something in it that differs between different cars…?

2012 GT, Calimer MT82, 3.73 gears, brembo fronts, shelby rotor upgraded rears, forgestar CF5 wheels (20x9 and 20x10.5) It's a premium, only options were HIDs and the gear.

 

[Alatalo]

I/We let the customers make the decision on there own and we document accordingly on the Repair Order. We show them the calculator which clearly shows a difference.

I did see your post in the ”what have you done to your Mustang” thread from the other week.

I know there is more behind the selection of dragrace tires, but is that by any chance the recommendation you give to dragracing customers…? 26” front runners combined with 275/50-15, to keep the diameter difference to a minimum…? And then obviously 28” front runners combined with 275/60-15…?

 

[07 Boss]

I ran a 28.8" wheel in back while I ran the stock size up front with no issues.

 

[skwerl]

As far as ABS and revolutions per mile, width plays no part. You are looking at the wrong measurement. What matters is tire height so the revolutions per mile are about the same. If you do some digging on Tire Rack's website looking at the specs on different tires, you will find the spec chart which lists, among other things, revolutions per mile for every single tire size for that tire. With a little time and effort you can find the appropriate front and rear tire sizes that have very similar height, and therefore similar revolutions per mile. I ran drag tires on my Bullitt and never had an issue with the ABS because I matched the overall tire diameter between front and rear.

 

[Alatalo]

I ran a 28.8" wheel in back while I ran the stock size up front with no issues.

Thank you,

The stock wheels in 2007 would have been 235/55-17 or 235/50-18, which means diameter 27.2-27.3”.

So a 5.5-5.9% diameter difference versus the 28.8” wheel in the back.

 

[07 Boss]

Thank you,

The stock wheels in 2007 would have been 235/55-17 or 235/50-18, which means diameter 27.2-27.3”.

So a 5.5-5.9% diameter difference versus the 28.8” wheel in the back.

You know what though, I just thought about it, might make a difference. I've always had a traction control monkey turn off my traction control every time I start my car so it is basically always off with the ability to turn it on if I need it. So I'm not sure if the tire difference is picked up by the traction control. No issues with the abs though.

 

[AdamBorz]

So just had my car in for inspection garage said front tires need replacing to pass. 245/50/18 Nitto in front and 305/35/18 Mickey Thompson SS in back. Looked it up actually front tires are larger than rear diameter wise… what do you guys suggest I go with? I bought the car this way BTW

 

[AdamBorz]

Closer

 

[AdamBorz]

No thoughts on going smaller diameter in front or larger in rear? I can go to a 19” wheel/tire 305/35/19 in the rear which will fill in the wheel well more? Or just leave well enough alone and replace front tires with the same thing??

 

[MADGT]

Well, heck, I am a sucker for beefier tires in the rear...so here's my set up:

18x10 F (285/40/18) and 18x11 R (315/40/18) --with a 3.3% difference front to rear. I've had no issues.

 

[AdamBorz]

Well, heck, I am a sucker for beefier tires in the rear...so here's my set up:

18x10 F (285/40/18) and 18x11 R (315/40/18) --with a 3.3% difference front to rear. I've had no issues.

Nice and shiny… I ended up ordering the 245/50/18 Nittos for the front and am switching to 305/35/19 in the rear. Same width, 1” bigger…

 

[MADGT]

Would love to see pics when mounted...

 

[AdamBorz]

Would love to see pics when mounted...

I don’t know if I could fit your tire size which are roughly 1.5 inches taller than what I have now. But that would have been another option as your tires are like .5” wider. My fronts are pretty slammed in the wheel wells, don’t rub either.