sprins

Hi,

My 19" tires have load index 96Y and 98Y (F R) which the SL apparently needs.

20" tires seem to only come in 92Y and 95Y (F R) which seems too little.

What gives? I have a set of 20" wheels which I want to install someday but I'm having trouble finding proper tires.

I searched the forum for "load index" to little avail. Since there is a lot of 20" in this forum I thought to ask your advice on this issue.

What to do?

sprins

If I ask the question the other way around: if you are running 20" wheels on your SL, what tires ate you using, and specifically what load index are they?

Appreciate the info.

threeMBs

Good question. Too bad most people - running stock dimeter 255/30-20 (92) front and 295/25-20 (95) rear - ignore it completely.

245/35-20 (95 load) front and 275/30-20 (97) rear (or even 285/30-20 (99)) can (and should be) safely used on any SL.

Edit: BTW, you can also use 265/30-20 (not as readily available, but with load of 94) front and 305/25-20 (which many use on SL and CLS, rated at 97).

overZealous1

do you plan on towing?
more air capacity generates the higher load rating. just bump your air pressure a little, or get the tires in the sizes listed above.. most high performance tires are designed to hold more so you can overinflate slightly.

sprins

Thanks guys. You just made me understand this load index thing.

Being able to floor it on the Autobahn from time to time I'm keen to respect the tire requirements. Instead of looking to 255/30ZR20 and 295/25ZR20 I'll be looking at the 245/35ZR20 and 285/30ZR20.

I believe the rear rim I have is 11J, so 275 might be stretching it a bit.

sprins

This is really useful: http://www.tyresizecalculator.com/

Fiddling around I get these options:

245/35 ZR20 and 285/30 ZR20 (diameter diff. 0.29%)
or
255/35 ZR20 and 295/30 ZR20 (diameter diff. 0.85%)

My guess is the 1st option to prevent rubbing? I don't want my fenders bending at the first curve... I know it also depends on the ET, which I think is conservative on my wheels (I'll have to check).

overZealous1

ah, the autobahn. now i understand your reason for asking. the load rating is not the only consideration for high speed. with higher sidewalls and moderate pressures, at high speeds you can get a constant wave in the tire. the bottom of the tire deforms to the road under the weight and at speed doesn't have enough time to return to normal shape before needing to deform again. this creates alot of heat in the tire. as a result, you can have the tire delaminate. most high speed runs will require you to set the tire pressures pretty high so you can lessen the chance of this deformation happening and tire heating up. sidewall stiffness plays a big part in this too.
if you are just doing a sprint up to speed and back, or keeping it under 110mph for long periods, it wouldn't be much of a concern. but, higher than that for long periods and i would really make sure you have the right tire. load rating is not really what you are looking for. each tire has it's own different construction and sidewall stiffness can very greatly between high performance tires. load rating can effectively be adjusted via air pressure.
this is really one of those times to not skimp on costs.

sprins

It's not just the Autobahn. My car is derestricted as well, so indeed I wish not to take chances. Safety first!

I saw that the diameter of the wheel cannot increase too much due to the support/control arm of the front suspension system.

And thanks for making me understand why tyre pressure is supposed to be higher when doing serious high speed travel. It says so in the fuel flap, but it didn't sink in why.

With this information I shall contact my tyre guys (who are Brabus dealer come to think of it now) and will ask them which tyre they might suggest.

threeMBs

That is why 245/35-20 is prefered over 255/35-20, as the later one will rub.

threeMBs

If your rear wheel is 11" wide, then even 285/30 won't do - for most 285/30s 10.5" wide wheel is the widest recommended. So 305/25-20 rear with correct load index of 97 will work fine (depending on your wheel's offset), but then 245/35-20 tire most likely should not be used as there will be a great front/rear diameter difference. Hence with rear 305/25, 265/30 front is best used (265 has higher load index over 255, provided of course your front wheel is not narrower than 9" wide, but 265 is harder to find to match 305).

sprins

I see... Hope this puzzle doesn't give me any headaches!

hostetlgaret

the bottom of the tire deforms to the road under the weight and at speed doesn't have enough time to return to normal shape before needing to deform again.

sprins

Well, the more I look into this 20" thing with the wheels I have on hand, the more I reckon these wheels are meant more for aesthetics than for performance. Which needn't be a problem as long as one is aware.

Since the wheels have 'conservative' ET values I'm hoping that 305 will fit on rear without rubbing as the rears are indeed 11J

Front: 20x9 ET31
Rear: 20x11 ET35

A fitment of 255/30 and 305/25 would be ideal. And considering that a NARDO speed-run is out of the question I'll look at the more economical tires. When making long and fast trips I'll have to install the 19 inchers again.

Probably Hankook or Yokohama or PS2s will suit my needs. I'll have a talk with my tire folks. Installation will be after the winter season, but if/when installed I shall report back.

Thanks for your attention as this was very informative to me. And I rather buy tires well informed...