amongmany

Ok, I've noticed a lot of guys running 17" wheels in the rear are still running 245... rear tires. Does this mean that the contact patch they're getting isn't any wider than the stock one?

xviper

This is my take on it:
The OEM 16" SO2s have a "sectional" width of 225 (mm), BUT it actually has a tread width closer to 245. Any other non OEM tire marked as 225 with not only have a sectional width of 225 but also a tread width closer to 225. So, in order to approximate the tread width of the OEM tire (16"), all other tires need to be 245s regardless of them being 16", 17", 18" or whatever people put on the car. This is why Honda went with 245 tires on the '04. They are not using any "oddly" made tires like they did in previous years.
"Contact patch" is all of the tire rubber that touched the ground at any moment. Just imagine a car with its tire parked on top of a piece of thick glass and you are under that glass looking up. What part of the tire is actually touching the glass?

amongmany

Right, I understand that the OEM 16's are really closer to 245, but does that mean that if I have a 17" wheel with a 245 on it as well that I have gained no traction advantage over my OEM wheels?

xviper

This is correct. You have more or less the same width of tread on the pavement. (An OEM tire with about 245 mm tread width will have about the same 245 tread width of the 17" tire. "245" is in mm and is the width of the tread. This does not change just because the size of the hole in the middle of the tire changes.) The shape of your contact patch may be slightly different because of the aspect ratio of the tire, but width-wise, you would be about the same. I say "about", because different makes and models of tires will have a slightly different tread width for the same size marking.

amongmany

Thanks, as usual, xviper! I appreciate your insights!

One more question: Is there any advantage to the upsized rims if you're not gaining any contact patch? I suppose you might save some weight if you buy the right rims, but anything else I'm missing? I always thought that one upgraded to gain traction, but now it appears that the most contact anyone is gaining by upsizing to a 17" rim is maybe a centimeter (for those that are running 255's rear).

xviper

When you go to a larger diameter wheel as in from 16" to 17" or 18", you should be going to a lower profile tire such that the wheel/tire diameter remains the same. By doing this, you gain a bit of rigidity in the tire sidewall (shorter) and thus, have less stress. The tires would then be more "sturdy" when going around aggressive corners. A change in traction will usually be only affected if the tread width changes.
Larger diameter wheels tend to spread its weight further away from the center of rotation, thus requiring more power to turn them. However, because aftermarket wheels can be made lighter, the extra mass further out is not so significant. If a 17" wheel were made of the same stuff as a similar 16" wheel, the larger one would be heavier and with more of its weight further from the center. As I'm sure you know already, it is easier to turn a smaller wheel than it is to turn a larger wheel. Same applies for when you are trying to affect a change in that wheel's speed, both accel and decel.

Jim@tirerack

Also the shape of the contact patch changes as you go to a lower profile tire. The shape is wider with a lower profile tire even if the width of the tire is exactly the same.

Jim

FF2Skip

[QUOTE]Originally posted by Jim@tirerack
Also the shape of the contact patch changes as you go to a lower profile tire.

Wildncrazy

One other caveat, the rated size is really only a suggestion. The tire manufacturers are given a lot of latitude in actual size vs. rated size.

You will find wide variances in actual widths with Michelin/BF Goodrich being narrower than most. Check to see the real width and not just the rated width.