Since most of you guys/gals have changed your rims ive got a question.

Ok ive been having some debate with someone regarding rims. I have never heard this before but it appears that my friend thinks that if you go with wider tires on your car it will slow the car down considerably. Im not just talking wider tires on your stock rims im talking about going from your standard 16" rims/tire to an 18" setup, say 8.5 and 9.5. I personally think it has nothing to do with the width of the tire, just the weight of the bigger rims and tires. Can anyone clarify?

I think whoever told you that is probably thinking....ummm....the wheels are bigger, the tires are wider, everything is heavier, then it must slow your car down. To some degree, he is partially right. But if someone is upgrading from 225/45/17 to 245/40/17, the car shouldn't be slower because the wider tire has more grip. The one small factor will be the 245 tire is heavier than the 225.

it may slow it down a bit, but with c classes other than the c32 i dnt think u would even notice it/or matter. i ran 225/40r18s for about a year, then replaced to wider rear wheels and tires....255s, i didnt notice anything except for improved rear wheel grip on corners.

If your car doesn't have enough HP to skid your tires on full acceleration, then I think thicker tires will only slow down acceleration.

Going from 16s-18s will:

1. Slow down your accelleration - Nothing too noticeable
2. Make your speedometer inaccurate...by 1-3 MPH, nothing major.

It's not like your car is going to be as slow as a Geo Metro...you'll prob. like the look over the minimal performance drop.

Erik

Yes, a wider tire will have a larger contact patch with the ground and therefore will have increased friction and rolling resistance. If this was not the case, then the roadholding would be no better when you go to a wider tire.

The increased friction/rolling resistance will worsen fuel economy and lower top speed somewhat.

Then there is the matter of the weight of the wheel/tire combination. The heavier it is, the higher the unsprung weight, which will actually counteract the enhanced grip and make the car a handful to drive on bumpy roads.

Normally it would lower top speed, but not in the case of our cars since they are limited by the factory we never hit true top speed.

Erik

Back to the physics class, this is related to the Moment of
Inertia. It takes more effort to rotate something further away
from the center axis.

From the center axis of rotation, the more cross sectional area of
material you have further out, the slower you rotate.

A well known example, ice skater pull in their hands to spin,
and stick out to slow down the spin.

Oh, yeah, you'll notice it, big time! Even between 225/45/17F+255/40/17R that I used in the summer and 205/55/16 all around with winter tires that I have on right now, the latter makes the car much peppier. You really feel the weight of these heavy wide wheels in the back.

if you go from 205/55/16 to 255/35/18, the overall diameter of the wheel is only 0.6% bigger. So if your speedometer reads 60 mph, the actual speed would be 60.4 mph. Nothing to worry about. This is a normal thing when going up or down in rim size, there are not too many combinations rim+tire that would keep the diameter exactly the same.