Mag1c Carp3t

Why do our cars have a higher front tire than rear. For example oem is 40F and 35R. I now have 19s and like the way the 30 looks on the rear. Can I put a 30 on the front with no issues?

chrisridebike8

It's to make sure the overall outer diameter is the same front and rear. 40% of 235 mm is roughly the same as 35% of 265 mm. That's why we run 235/40-18 and (I personally run) 265/35-18s.

QWKSNKE

Actually if you run the numbers with stock tire sizes, the fronts are around half inch taller than the rears. This is part of the reason I went with a 275/35/18 on the rear of my car. (Just a tad taller than the fronts) Makes the car look so much better from the side now

QWKSNKE

Instead of reducing the front, just increase your rear width to a 265 or 275 with the stock sidewall ratio. This will even your overall height to match the front

chrisridebike8

Yeah. Thats why I run 265s out back.

Diabolis

OP - I may be stating what you already know, but based on the way your question is worded, I suspect that you may be unaware that the sidewall height is expressed as a PERCENTAGE of the width, not as an absolute number the way the width and wheel diameter are. In other words, a 205/45 and a 305/45 for example are going to have very different heights - the first one is 45% of 205 mm (so 92.25 mm) while the second will have 45% of 305 mm (so 137.25 mm) - the first tire will be only 2/3rds as high as the second. That's why on a wider tire the height (or aspect ratio) drops - so they would be roughly the same height.

Was that perhaps the answer that you were looking for, or were you really asking about the ~5 mm difference between the OEM F & R tire heights?

If you run too high a tire at the back, at high speeds you'll lose the low pressure created by the airflow under the car and thus lose downforce (which if you track the car and have to slow down from 260 km/h is quite noticeable). Besides, with the OEM suspension and alignment settings, the height of the front and rear axles is NOT exactly the same - when I put my 235/40-18 snows all around, the rear of the car does sit higher than the front by a small margin.

Mag1c Carp3t

Yes that makes sense. I was curious why our car does this. I came from a car that didn't run staggered profile only staggered width. How much of a diameter difference can the tcs system tolerate? Has anyone run a matching profile front and rear?

tpliquid

if u run matching profile front and rear u gona have 1 inch difference. . or run 235/35 with 275/30 will have super close diameter

list below

235/35/19 = 25.47 inch diameter
275/30/19 = 25.49 inch diameter
265/30/19 = 25.25 inch diameter
255/30/19 = 25.02 inch diameter

275/35/19 = 26.57 inch diameter
265/35/19 = 26.03 inch diameter
255/35/19 = 26.03 inch diameter

Mag1c Carp3t

How would the car react to the difference, I know from experience the TCS system can act up? I can always adjust the height of vehicle if under car airflow is an issue.

tpliquid

i dont understand what you are trying to do? Are you trying to get the same diameter front and rear? same look of the sidewall front and rear? or you just want to have more sidewall in the rear than front?

Mag1c Carp3t

Yes I am trying to go same 30 profile front and rear for a matching look. It'll also allow more clearance in the front to go wider. I am just concerned in how the car will react. I know there is a certain tolerance the car handle in difference in diameter.

tpliquid

then go 235/35 and 275/30

it will give you the CLOSEST profile off by .02 inch

what is your tire sizes right now?

Diabolis

Correct - the TCS (or on an AWD car the center diff) monitors wheel speeds and expects to see the front wheels spinning at the same rate as the rear wheels. I don't know what - if anything - the TCS on the C63 would do when it detects that the fonts are spinning faster than the rears. Maybe it will apply and drag the front brakes, maybe it will partially RELEASE the rear brakes during a panic stop (which would IMHO be the biggest concern and outright dangerous) or maybe the TPMS will tell you that you have two flat tires on the front...

tpliquid

i calculated diameter difference on stock tires is 1.7% SO i guess it will let u get away with +/- .1 % or more

QWKSNKE

You are focusing on the wrong numbers.

You can't use the same '30 profile' front and rear to even out the correct look without changing the width of the tires substantially. (To the point of the tires will either be to narrow or to wide to fit on the wheels)

Always focus on the total diameter of the wheel and tire together as tpliquid referenced a couple of posts above. In order to get correct diameter front/rear combination in a staggered wheel car that has limited width tolerance to work with you will always need to combine different ratio tires.

By the way, you said you updated to 19" wheels, what tires sizes are you running now?

shamrode

Hey guys. I just slapped son Cup 2's with new wheels on my car. 2009 stock suspension.
Front - 235/35/19 offset 38
Back - 275/30/19 offset 43

I am loving it; however at high speeds (only at high speeds) on the freeway when there is a dip in the road the back tires rub for a second. This doesn't happen on bumpy roads or speed bumps; only when driving at highway speed and encountering a serious dip in the road. Is this ok?

Diabolis

That is entirely up to you... if you're OK with the tire occasionally rubbing against the fender and damaging both the fender and the tire (and possibly risking a blowout if you cut the tire on the fender when you hit a decent-sized dip at speed), then it's OK with the rest of us.

The offset on your rears is a too low for a 275, especially the PSC2 which has pretty square shoulders & stiff sidewalls and tends to run on the wide side like the PSS. You'd need rear rims with offsets of ET50-52 to accommodate a 275 without rubbing.

shamrode

Sorry the rears are 265 not 275. Are my offsets ok?

chrisridebike8

You are rubbing! No. Your offsets are not ok. Rubbing is not normal and not really safe.

shamrode

OK Ok; dangerous. Got it. Will I rub with a 255/30/19 and 43 offset?

chrisridebike8

We need to know wheel specs. Tread width doesn't help us that much unless we know wheel width in addition to the offset.

shamrode

OK. Front - 235/35/19 wheels offset 38 and width 8.5; there is no rubbing.
Rear - 265/30/19 wheel offset 43 and width 9.5; rubbing only on road dips at high speed
If I change the rear to 255/30/19 (same width/offset) will there still be rubbing?

QWKSNKE

What part is making contact? Fender lip or inside the fender well?

jay rick

Not sure if this is safe, but how about thinning your wheels hug by 2 mm that will give you a 45 mm off set. 255 on 9.5 doesnt look good IMO.
But if that is not recommended, you will have to run higher offset.

shamrode

The fender lip. I don't understand "thinning wheels."