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I have read a variety of threads/posts on the subject of the tire sizes and the fact that its necessary to maintain a difference between the front and the rear (rear larger) to keep the computer happy. Some indicate that the front must be a certain percentage of the rear... Or at least within a certain range.... Other threads indicate that as long as the rear tire is larger, then all is well.
My car started life as a ‘17 base model with the 18/19” set as below:
^^ Nearing a square setup with 265 front and 285 rear.... and the rear still 2.3% larger than the front in overal diameter. Is that enough for the computer to be “happy”?
275/285 and rear is 1.1% larger in overall diameter.
There are a ton of other variations - especially when considering changes to the rear tires too.... I’m just interested in the percentage between front and rear.... What’s permissible? Is there some magic ratio? Or is it “ok” as long as the rear is larger by a “smidge”?
What say the folks smarter than me?
Last edited by malexand123; Dec 20, 2018 at 10:32 AM.
I suck at attachments apparently, so I'm doing the attachments from my original post above... Just makes it easier to see them instead of clicking and going to a new window.
I have read a variety of threads/posts on the subject of the tire sizes and the fact that its necessary to maintain a difference between the front and the rear (rear larger) to keep the computer happy. Some indicate that the front must be a certain percentage of the rear... Or at least within a certain range.... Other threads indicate that as long as the rear tire is larger, then all is well.
My car started life as a ‘17 base model with the 18/19” set as below:
^^ Nearing a square setup with 265 front and 285 rear.... and the rear still 2.3% larger than the front in overal diameter. Is that enough for the computer to be “happy”?
275/285 and rear is 1.1% larger in overall diameter.
There are a ton of other variations - especially when considering changes to the rear tires too.... I’m just interested in the percentage between front and rear.... What’s permissible? Is there some magic ratio? Or is it “ok” as long as the rear is larger by a “smidge”?
What say the folks smarter than me?
I ran rear tires 1 inch taller than stock with stock fronts. Track and all. No problem. Some here run 315 front and 335 rear with no problem at all. I don't think anyone has reported any computer issues due tire size.
I ran rear tires 1 inch taller than stock with stock fronts. Track and all. No problem. Some here run 315 front and 335 rear with no problem at all. I don't think anyone has reported any computer issues due tire size.
Stock tires are .9" different, which is 3.5% or a 1.034 ratio - I would try to stay as close to that as possible.
I've hear the 4% rule repeated before just not sure its set in stone.
The rules were more of an issue for C5/6. C7s are much more tolerant to different diameters. Even when I run a square set, the traction control was a bit sensitive especially in touring mode, but there were no other issues.
Stock tires are .9" different, which is 3.5% or a 1.034 ratio - I would try to stay as close to that as possible.
I've hear the 4% rule repeated before just not sure its set in stone.
But has someone actually proven that hypothesis on the c7?
For track, looks like you can do whatever you want. Stability control seems to work with anything, and traction control doesn't cause any trouble in PTM or competition modes.
For street, TC gets more sensitive as you get closer in diameters in Touring and similar modes. When I used a square setup, it was pretty proactive in intervening. The factory setup is 25.8" / 26.6", which is 0.8". If it were me, I suppose I'd also want to have at least 0.3-0.4" difference so everything would work fine. I have to say, it's never like C5/6 that flat out caused trouble, and even threw codes.
For track, looks like you can do whatever you want. Stability control seems to work with anything, and traction control doesn't cause any trouble in PTM or competition modes.
For street, TC gets more sensitive as you get closer in diameters in Touring and similar modes.
Interesting... I wonder how the computer is sorting things out. In PTM mode it kind of makes sense as TC is semi-off or at least has wider tolerances in the sense that it allows for some wheel spin. I would guess that with SC the yaw sensor is comparing vehicle rotation against tire speed from side to side vs front to back. My experience with other cars is once you get a large enough difference between front and rear the TC becomes overlay sensitive. Glad to hear the C7 doesn't go crazy like that to allow a wider range of tire choices.
So, I don't know where to start, so this may look a little bit like a rambling. But if you're willing to read through it there might be a few nuggets you can use.
First, each and every tire has a typical "range" of rim width you can use. For example, a stock 245 width tire probably will require a 8"-10" range wide wheel for fitment, but has a nominal width of 8.5" as measured for its width. You can't really go outside of the range as suggested by the manufacture, I mean, they'll likely "fit" but they won't work right. There'll be too much stress put on the sidewalls for real world operation.
So in reality you're LIMITED in what width tire you can actually run based on the current wheel you have. If your current setup is on the 245 wide 19" tires, running on a 8.5-9" wide rim, neither the 265 or the 275 wide tires are "ideal" for the fit. I would check the specs on the tire manufacturer's website first for those specific tire sizes and make sure your rim width falls well within the recommended range. For example.
Now, you listed 2 additional sizes that have the same 35 ratio sidewalls, where in reality you should step down in sidewall ratio to maintain same sidewall height. Keep in mind that 2nd set of number in a tire's size is a ratio of the sectional width (a 275/35/Rxx tire means the sidewall height is 0.35 x 275mm, which is 96.25mm or 3.79". The factory sidewall height is 98mm, or 3.85" tall, but on 18" rims). To maintain the same or nearly the same sidewall height to the 245/35 (85.75mm) ratio on 19" rims you should be looking at 275/30/R19 (82.5mm) or even 285/30/R19 (85.5mm). Obviously at 285/30/R19 it eliminates ALL the stagger.
I'm not going to comment on how "sensitive" the traction control system is to front/rear wheel size differences and what percentage you can get away with without triggering ABS and PTM interference. What I will add is, you're adding nearly 10mm of tire height in the front, or nearly half an inch of ride height adjustment. When I had my other car corner balanced, 2 turns on the height adjustment collar, which amounts to a couple of mm, made a HUGE change in shift in weight distribution. Half an inch higher in the front will dramatically change the way the car will handle would be my prediction.
What's more important, actually, is the LOAD RATING of the tire you're looking at (the 2-3 digit number following the size in parentheses (96Y, for example). You want the load rating to at least match and/or exceed the factory load rating on the OEM tires.
On my 2014 z51 i have 245/35/19 up front and 305/35/20 in the rear i have all abs codes, service stabilitrak, service axle, etc. Will a 315/35/17 weld in the back clear the lights or will ihave have to get new wheels and tires up front? Thanks in advance i have been dealing with this issue for months
On my 2014 z51 i have 245/35/19 up front and 305/35/20 in the rear i have all abs codes, service stabilitrak, service axle, etc. Will a 315/35/17 weld in the back clear the lights or will ihave have to get new wheels and tires up front? Thanks in advance i have been dealing with this issue for months
I doubt you fit a 305/35/20 out back without issue. You're saying that tire size set off all those check engine codes? Sure it wasn't coincidence ?
Last edited by BrunoTheMellow; Dec 21, 2018 at 04:33 PM.
Have you tried putting a normal size on to verify they go away? That's 1.7 inches taller than stock. I've gone with 285/35 which is 1.2 inches taller than stock with no problems.
i havent tried putting stocks back on but i know the 305 in the rear is my problem im trying to see if a weld setup in the rear would make this issue go away cause i was going that route anyway
It’s not the width that’s the problem it’s the series, IE 35 vs 30 depending on wheel diameter which effects oveall tire height. For example a 305 / 30 - 19 should work as it’s very close to the OEM height at just 0.5” shorter.
Front/Rear Tire Size Differences
