[APKhaos]

There I go, understating again! OK, it most definitely will effect gearing.

Manging effective diameter is the most frustrating part of tire selection. Everyone understands the width aspects of tire selection, but the diameter aspect is even more important.

We know that tires of a given size - say 245/45 x 16 from different manufacturers will be different absolute dimesions, right? A Proxes RA-1 adn Kuhmo Victoracer [and any onther tire] in this size will each have different dimensions.

Differences in section width can mean one brand fits your car fine, another brand rubs badly. It happens. In part, this is because the sectional width differences between brands, but it can also be caused by differences in sectional shape, which is harder to predict from the published specs. The classic case is one tire that fits on you fat rear Fuchs without a problem, and another brand - same size - that rubs itself to destruction on the spring plate bolts. This happens too!

Differences in rolling diameter are more subtle. While diameter can cause fit problems, it is the rolling diameter that can kill gearing.
First step is to figure out what the stock tire diameter is for you car. Look that up in the tire specs for the stock tire fitted to the car.

For example, lets say the stock tire on the stock wheel has a diameter of 24.5", which happens to be about right for the rear tire on a mid 80s 911.

If you look at the specs of the tire, you can quickly calculate the circumference of the tire to figure out the number of tire revolutions per mile. Tire vendors often show this figure in their spec charts - less math for us! For our stock 24.5" diameter tire, thats 853 revs per mile.

Now, lets say we want to go to a wider 16" wheel with a 265 tire, and we are looking at the two candidates I mentioned above:

Stock: 24.5" diameter, 865 revs per mile
The Victoracer: 25.3" diameter, 820 revs per mile
The Proxes RA-1: 26" diameter, 815 revs per mile

As you can see, each of these tires is result in significantly less revolutions per mile. That means that you have a higher final drive ratio than the stock setup. Its like fitting a taller ring and pinion.

In some cases this might be good, but in most cases it is not. Its best to keep as close as possible to the stock diameter to retain stock final gearing with your new wheels and tires.

This is especially important when you are increasing the diameter of your wheels as well as their width. So many ricers make the mistake of fitting larger diameter wheels with low profile tires [that have a larger overall diameter] and then wonder why their car is slower. They increased their gearing without ever knowing they had done it!

This is something to keep in mind when changing wheels and/or tires, or even when simply changing from one brand to another - say from streets to DOT R compound tires. Check the dimensions to be sure you aren't hurting your gearing in the process.