15" vs. 16" Fuchs?

[smross]

How much of a reduction in gearing is there when going from 16" to 15" Fuchs with stock 205/225 tires on an SC. Also, is there a seat-of-the-pants difference in acceleration?

TIA

[911Velocity]

It depends on the size tires. Find out the diameter of the rear tire (tire rack has this on their website) and then compare the two. The smaller diameter tire will provide better acceleration, how much better depends upon the difference which will probably be negligable.

[RaceProEngineer]

SM Ross,

There will be a slight difference in final drive ratio between 15" diameter wheel and the 16"er, PROVIDED you are speaking of the same brand, model, and number tire. On the Autocross and Racing board a short time ago, there was an extensive discussion of BIG differences in SIZE between, say, a Dunlop 8000 225-50-16, and a Goodyear GS/CS 225-50-16.

The second part of your question, regarding noticable acceleration, can best be answered if we knew the CIRCUMSTANCES under which you are accelerating. Here's why:

Theoretically, ALL OTHER THINGS BEING EQUAL, the smaller diameter wheel/tire combination will give better low-end torque, resulting in quicker acceleration. But remember, the 225-50-16 rubber is going to be WIDER than the 225-50-15, providing slightly more traction. So if your car is "rear grip limited" (has a reserve of power, can readily spin the rear tires), then you may achieve better results with the larger (wider) tires.

We have also found that the larger diameter wheel/tire offers a smoother transition from extreme cornering to extreme acceleration, as in road racing. However, a 225-anything on the rear of a 911SC is going to be sadly inadequate for this application.

Hope this helps.
Ed LoPresti
RacePro Engineering
New York

[Jack Olsen]

As has been pointed out here, it's not the size of the wheel, it's the outside diameter of the tire you run on it.

If you were running 225-50-16 tires in the back, and are switching to 225-50-15's, for example, you'd have a 1 inch decrease in outside tire diameter. Your speedometer would be off by 4%, and you'd see a slight (maybe 4%?) increase in effective torque from the lower gearing. Your top speed would be reduced, as well.

It you were considering going to a 225-50-15 tire when you were already running a 225-45-16, the difference would be only a tenth of an inch, and a speedo/torque difference of less than 1%.

As you can imagine, it's cheaper to go to the shorter tire size than it is to buy a new set of 15-inch wheels. Tire size availability is the limiting factor no matter which route you take.

This tire size calculator is a good tool.

[lateapex911]

Also, probably not an issue for most 911s, but when a larger diameter tire can limit the amount of lowereing done on certain cars. Going to the different wheel size can lower the CG in cetain instances.