I test drove a car today on 21 inch wheels. The car drove great at all speeds and caused no issues at any speeds wheen driving normal, but from a third gear roll (almost wot) it had this jerky in and out of throttle type feeling, but less exaggerated than actually being on/off the gas - more like a bit of throttle cutting in and out feeling(?) The owner told me that was because of traction control being on. I currently have a c63s and it doesn't act this way when traction is on. So I switched it to sport traction and it still did it (sport traction will allow some spin in my c63s before traction intervenes) . The owner suggested that it's because it's a couple degrees above freezing, slightly damp roads and on summer pirelli tires, so the traction is causing it. I turned off traction fully for a third pull and it did not do the weird jerky in/out cutting throttle feeling again. I figured he's probably right and maybe the traction control acts different than in my RWD car.

I'm currently sitting at home thinking and I remember checking the tires sizes, I'm almost positive they were 265/30/21 (not 35) and 295/30/21. If the car drove highway speeds without doing this, but did at pretty much full throttle from a third gear roll, would this diameter difference be the cause? A tire calculator shows a roughly 2.53% difference.

And finally, regardless if this is the cause or not, and knowing it prob should have a 265/35/21 front instead, would that tire difference cause any damage to the drivetrain. Again, it never does it when driving normal, at any speed, or throws any codes or anything.

Thanks in advance.

Edit : ive been doing lots of searching on the forum and the only stuff I can find is in the w212 section, but it appears within 3% is fine?
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the 265/30/21 is nearly the same as 265/35/20. But the rear (295/30/21) posses a deviation of 3.6% variance from factory setting which may cause some issue.

Test driving the car at upper limit in summer tires in cool/damp weather is certainly not ideal. Such high performance tires will get no traction off the ground and therefore, it could result the car engaging some traction control issue.

 

I chatted with a representative from Mercedes online, on their own website, to ask what the tolerance was for tire diameters. I needed to know if the 19" winter tires I was ordering were within spec. They told me the overall diameter deviation front to back needs to be within 3%. Going beyond that could cause high wear or damage to the transfer case/clutch pack, and I'm sure it could also effect the calibration of traction/stability control.

 

If the car was limiting due to TC, you would be able to see the icon flashing on the dash. It is the yellow car with wavy skid marks next to it.

 

And this was specific to the w213 e63s when you asked? If so this is info I was looking for, so if I buy the car I can keep it on the current tire setup.

I have a feeling it might be tune related, its got a renntech tune on it. Id actually like it flashed back to stock anyways before I have another company tune it down the road when it had downpipes . Would give me a chance to get some stock dragy baselines anyways.

I don't believe it matters what the stock tire difference is when comparing diameters, what would matter is the new diameter difference from front to back on the new tires.

 

I was thinking about that too as my car does the same but at the time I didn't think to look for it.

 

the deviation is the speedo. not the diameter I am referring.

 

Oh by bad, I'm not too worried about that. I'll GPS the car and see how far off it is and try and keep that difference in mind when I'm trying to do the speed limit.

 

Your front speedo will read factory calibration while your rear will read 3.6% off from factory calibration which the other poster from above has indicated it can throw codes into the car’s computer and could create issue with the awd and other sensors perhaps. So, just keep in mind.

 

Your safest bet is to change the front to 265/35/21 to preserve the same ratio as before. However IDK if that size presents rubbing issues when turning wheels all the way.

A second option (less ideal) is to try 305/30/21 in the rear and it will get you a lot closer to the current ratio. But that presents a risk of rubbing on wheel well when hitting a bump if it sticks out too much.

 

305 won't fit with the aftermarket wheels' offset, just judging by eyeballing it.

For whatever it's worth, it was a dry/near freezing dday today, so I took the car for a full tank of 93 (vs the crap ethanol free 91 it had in it), and the car didn't make this feeling at all after several actually full throttle attempts -even with traction full on. Tried swcomd and theirs hear, traction on and in sport. Nothing weird. This car is also on a renntech tune btw. I was also just thinking it snowed in the morning for a short period and then melted so there was lots of grit, sand and salt on the road that could have attributed to it as well.

I'm not too worried about it anymore but I should maybe think about changing the front tire. I assume the rears being 75 percent tread and putting on brand new front tires that are closer on dimater is still less of a difference then the current setup.

 

3% is acceptable and normal to where modern differentials/gearing can easily tolerate it.
I like to be as close as possible (<1%), I run 265/35/20 (20x10) and 305/30/20 (20x11) wheels.

 

If you change your current 265/30/21 to 265/35/21, then you will have a speedo variance of 3.5% ... at least it will have the same speedo variance as the rear.

 

I'm pretty ocd and knowing that it's not as close as it should be is going to bother me, but as long as it's safe enough to not damage the car then I'll likely just wear them down a bit and replace them with better sizing (likely by summer) . It's on pirelli tires anyways so I'd like to get some ps4s on it.

👌

 

Another question, on freezing temp pavement, is it normal for the right rear tire to feel like it skips a bit on 90 degree turns? When I get on the gas to turn right or left onto a street I notice the inside rear tire seems to skip or crab a bit. Probably fighting for traction. This is my first awd car so it seems weird to me.


Edit : talked to a couple friends with them and they suggest a summer tire compound in winter seems to do that? It's on summer pirellis

 

I've been thinking about this more and I'm almost thinking as a piece of mind maybe I should switch to a 265/35/21. The rear tires are roughly at 75% remaining if I had to guess, so the new fronts wouldn't be matching in tread depth but at least they'd be closer in ratio. I'm noticing in sport traction this car cuts power in in an on/off jerky fashion with much less throttle input than my RWD c63s. I'm wondering if the 2.5% difference affects that? My mercedes tech friend doesn't believe so, as he said if the diameters were too far apart the car would throw codes for it pretty fast (wheel speed?). Is this a good move or am I being OCD.