Tire Size Tolerance
Thread Starter
Instructor
Joined: Jun 2021
Posts: 129
Likes: 71
From: Wiesbaden, Germany
Tire Size Tolerance
I know this has been brought up a lot but there are so many different answers and not a lot of info for base cars.
I picked up new rims, AR924, 19*8.5 and 20*10. I would have preferred bigger rims, but the cost of shipping and/or finding someone to ship was too much. I am looking for new tire sizes. I did a bunch of math, well Excel did, and I am just wondering how much tolerance is built into the different tire sizes.
I do live in Germany, so I do drive fast. My average cruising speed on the Autobahn is 225, 140mph. I don't want to be going 140+ plus and have traction control do funny things.
Stock Front – 245/40/18 – 25.71875”
Stock Rear – 285/35/19 – 26.875”
Different 1.15625” or 1.044%
I am looking at sizes
245/35/19 and 295/30/20 = 1.1875” or 1.046%. That is 1mm difference, shouldn’t be a problem
Now if I got to
255/35/19 and 295/30/20 = .9375” or 1.036%. That is about 7mm difference. Is that enough to cause issues?
I have seen people have issues with 1.02%. I don't know about 1.3%
If I could go to 305 on the rear, I would be good, but the tire is not recommended for 10” rims, so that is a no-go.
I picked up new rims, AR924, 19*8.5 and 20*10. I would have preferred bigger rims, but the cost of shipping and/or finding someone to ship was too much. I am looking for new tire sizes. I did a bunch of math, well Excel did, and I am just wondering how much tolerance is built into the different tire sizes.
I do live in Germany, so I do drive fast. My average cruising speed on the Autobahn is 225, 140mph. I don't want to be going 140+ plus and have traction control do funny things.
Stock Front – 245/40/18 – 25.71875”
Stock Rear – 285/35/19 – 26.875”
Different 1.15625” or 1.044%
I am looking at sizes
245/35/19 and 295/30/20 = 1.1875” or 1.046%. That is 1mm difference, shouldn’t be a problem
Now if I got to
255/35/19 and 295/30/20 = .9375” or 1.036%. That is about 7mm difference. Is that enough to cause issues?
I have seen people have issues with 1.02%. I don't know about 1.3%
If I could go to 305 on the rear, I would be good, but the tire is not recommended for 10” rims, so that is a no-go.
Last edited by MKR_DmD; Feb 11, 2022 at 08:41 AM.
Race Director
Joined: Sep 2007
Posts: 16,557
Likes: 2,106
From: Peoria/Phoenix AZ
C6 of Year Finalist (performance mods) 2019
It seems nobody at GM either knows or (most likely aka legally CYA) will release the exact information as to when the nannies kick in due to non-stock tire sizes. However, it's a safe bet that the programmers would have made sure that the OEM tires would never exceed the limits. So as an individual, you can calculate the maximum and minimum limits for the OEM sizes and then then compare that to whatever brand or size tire you desire.
The calculations include knowing the OEM new tire and bald tire (tread depth) circumferences for both front and rear tires. Then find the high limit ratio for front to low limit rear tires and the low limit ratio for front to high limit rear tires. You can apply those ratio limits to whatever non-stock tires you desire, using the same factors. Your personal safety margin will be covered by the assumption that as a speed conscious driver, you would never drive at high speeds with tires worn beyond the wear bars.
The calculations include knowing the OEM new tire and bald tire (tread depth) circumferences for both front and rear tires. Then find the high limit ratio for front to low limit rear tires and the low limit ratio for front to high limit rear tires. You can apply those ratio limits to whatever non-stock tires you desire, using the same factors. Your personal safety margin will be covered by the assumption that as a speed conscious driver, you would never drive at high speeds with tires worn beyond the wear bars.
Thread Starter
Instructor
Joined: Jun 2021
Posts: 129
Likes: 71
From: Wiesbaden, Germany
It seems nobody at GM either knows or (most likely aka legally CYA) will release the exact information as to when the nannies kick in due to non-stock tire sizes. However, it's a safe bet that the programmers would have made sure that the OEM tires would never exceed the limits. So as an individual, you can calculate the maximum and minimum limits for the OEM sizes and then then compare that to whatever brand or size tire you desire.
The calculations include knowing the OEM new tire and bald tire (tread depth) circumferences for both front and rear tires. Then find the high limit ratio for front to low limit rear tires and the low limit ratio for front to high limit rear tires. You can apply those ratio limits to whatever non-stock tires you desire, using the same factors. Your personal safety margin will be covered by the assumption that as a speed conscious driver, you would never drive at high speeds with tires worn beyond the wear bars.
The calculations include knowing the OEM new tire and bald tire (tread depth) circumferences for both front and rear tires. Then find the high limit ratio for front to low limit rear tires and the low limit ratio for front to high limit rear tires. You can apply those ratio limits to whatever non-stock tires you desire, using the same factors. Your personal safety margin will be covered by the assumption that as a speed conscious driver, you would never drive at high speeds with tires worn beyond the wear bars.
Base car -
Stock rear – 26.875
Stock Front – 25.71875
Now if you minus 9mm from each one
Rear Bald – 26.52
Front Bald – 25.364
Now if the front tire is bald and rear is new the ratio is 1.059
Now if the rear tire is bald and the front is new, the ratio is 1.03118
I would call that the safe ratio.
The ratio I am looking at (255/35 and 295/30) is 1.0363 which is already 5.5mm low on the rear tire. So, I can only run my rear tire down another 3-4mm before triggering traction control issues, but that is if the front tire loses nothing, which I highly doubt. Since the tires I am looking at are only 7mm deep on the rear, I don’t’ think I will have an issue.
Racer
Joined: Apr 2009
Posts: 336
Likes: 60
From: Sevierville, TN
I am also in the "ratio wormhole". I can't decide what to get, for fear of the TC issues.
I have 19x9.5 and 19x11 wheels being built for my 05 Base "narrow body" and I am trying to figure the tires out in the down time.
I have found one thread for this exact sizing and they are saying 275/30/19 up front and 305/30/19 out back works. But what if I want a 315 or a 325 in the back? What would that translate to on the F/R aspect and such?
I have 19x9.5 and 19x11 wheels being built for my 05 Base "narrow body" and I am trying to figure the tires out in the down time.
I have found one thread for this exact sizing and they are saying 275/30/19 up front and 305/30/19 out back works. But what if I want a 315 or a 325 in the back? What would that translate to on the F/R aspect and such?
Pro
Joined: Feb 2017
Posts: 610
Likes: 79
From: Fallbrook SanDiego county
I've been working on this as well! On this older thread http://www.corvetteforum.com/forums/...els-on-c6.html Guys are pretty adamant about not having AH problems until you go over 1.06% so it looks like there is some room to wiggle a bit. I'm contemplating a 1.0305% & think that's the low end limit.
Race Director
Joined: Sep 2007
Posts: 16,557
Likes: 2,106
From: Peoria/Phoenix AZ
C6 of Year Finalist (performance mods) 2019
Another way to look at the front to rear ratio, is to compare tire revolutions per mile. Front 18" OEM base tires are 808 revs per mile, vs 774 for the rear 19" tires. (1.0439 to i)
Pro
Joined: Feb 2017
Posts: 610
Likes: 79
From: Fallbrook SanDiego county
Pro
Joined: Feb 2014
Posts: 714
Likes: 106
From: England
Active handling is the issue not traction control. I run 19/20's with a 1.035% delta but with coilovers and I`ve had a few `moments` with AH in turns where I`ve not really been pushing but the chassis is loaded.
My advice would be disconnect the steering angle sensor under the column. This kills the AH but maintains the TC control function.
My advice would be disconnect the steering angle sensor under the column. This kills the AH but maintains the TC control function.
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Pro
Joined: Feb 2017
Posts: 610
Likes: 79
From: Fallbrook SanDiego county
Active handling is the issue not traction control. I run 19/20's with a 1.035% delta but with coilovers and I`ve had a few `moments` with AH in turns where I`ve not really been pushing but the chassis is loaded.
My advice would be disconnect the steering angle sensor under the column. This kills the AH but maintains the TC control function.
My advice would be disconnect the steering angle sensor under the column. This kills the AH but maintains the TC control function.
Pro
Joined: Feb 2014
Posts: 714
Likes: 106
From: England
275/30 R19 325/25 R20
Fresh supersports at the time. In my case it set off a frankly terrifying front to rear to front bounce all through the bend. Had I been `going for it` I've no doubt it would have put the car off the road. Now the bouncing could be down to the coilovers and the spring stiffness change in my case more than the wheel diameters but I`'ve read enough horror stories on the forum to not really trust the system.
If you're a reasonably experienced driver the situations where the AH would cut-in on a standard car and help are not situations that the driver couldn't deal with him/herself. If you deliberately put the car into a situation where its properly sliding I found the AH made it much more snappy and unpredictable. You could also easily out-idiot the AH and the car would still spin. So for me TC gives you a good safety margin, in wet driving for example, and I feel better off without worrying the AH is going to do something stupid unexpectedly.
Edit to add, sorry my first reply should have been the ratio is 1.035, so 3.5% difference. Which may well be outside the system tolerance but even so I still prefer it gone.
Fresh supersports at the time. In my case it set off a frankly terrifying front to rear to front bounce all through the bend. Had I been `going for it` I've no doubt it would have put the car off the road. Now the bouncing could be down to the coilovers and the spring stiffness change in my case more than the wheel diameters but I`'ve read enough horror stories on the forum to not really trust the system.
If you're a reasonably experienced driver the situations where the AH would cut-in on a standard car and help are not situations that the driver couldn't deal with him/herself. If you deliberately put the car into a situation where its properly sliding I found the AH made it much more snappy and unpredictable. You could also easily out-idiot the AH and the car would still spin. So for me TC gives you a good safety margin, in wet driving for example, and I feel better off without worrying the AH is going to do something stupid unexpectedly.
Edit to add, sorry my first reply should have been the ratio is 1.035, so 3.5% difference. Which may well be outside the system tolerance but even so I still prefer it gone.