I just put new wheels and snows (Pirelli 240 Snow sport) on my new Cayman S. I was able to get a deal on wheels and tires except the tires on all of the rims were the same size (235-40-18). Of course, the stock tires on the rear of the Cayman are larger (265-40-18). The Porsche dealer I bought them from said that most people do this so they can rotate them each year (fronts to back).

Now, as I see it, the smaller tire on the back will now make the speedometer read about 3mph more than I am driving (3.6%, or actual = 57mph, speedo=60mph). In addition, the odometer will show more miles than actually driven by the same 3.6% or about 500 miles every 15,000 miles driven (actual miles=14,500; miles on odo=15,000).

This, of course, assumes that the sensor for the odo and speedometer are on the back wheels.

Since I am leasing the car, I am very interested this difference, if there is one.

On one thread I read that the Cayman reads the diameter each time the car is started and driven and adjusts.

Is this true? And, if not, can an adjustment be made to correct the error?

Thanks all.

BTY, I have driven the car in the snow and it works much better than expected.

I think no. I have no idea where the odo or speed sensors may be but it has nothing to do with section width. It has to do with circumference. There is a reason, for example, 19" wheels have lower profile tires. (aka +1 sizing).

If the original and snows are both 18" with 40 series tires, theoretically there should be no difference unless there is some difference in the circumference of the two tires.

Chows4us

There is a difference in circumference of 2.5 to 3 inches. When the wheels were off the car I noticed a large difference, so I rolled them on the ground and measured the difference. Of course, with no weight on the wheels the circumferences would be different, but I guess the % would be the same.

Here is the url of a calculator given in another post:

http://www.miata.net/garage/tirecalc.html

I stand corrected. Nice find. I never heard of the width having anything to do with circumference but I guess it does!

Your speedo will be slightly effected (the 3.x%), but the calculation given is not an exact science. The best way to tell and findout what the effect is would be to find out the exact O.D. of the tires by calling the tire manufacturers of the two different types of tires because a 265/40R18 made by Michelin will not have the same O.D. as a 265/40R18 made by Pirelli (or Bridgestone etc) and the same can be said of the snow tires. They all are very slightly different in size even though having the same size designation makes you think they are exact.

Before anyone questions me on this, I worked for a premium tire manufacturer for over 5 years in my past job.

Take care

NewellScott - You should give the dealer's service manager a call and ask them about this. I don't know that the speedo and odo will recalibrate with the new tires, but the service people at the dealer should be able to tell you one way or another.

The aspect ratio of the tire (30, 35, 40, etc.) is determined as a percentage of the section width, so the thinner tires will have a different overall diameter and rolling circumference, given the same wheel size and aspect ratio.

You can look up the specs (overall diameter and number of revolutions/mile) for most tires that are sold on Tire Rack by going to their Web site - find the tire brand and model and click on specs. That will give you some valuable info - you can use the two rev/mile specs to figure a percentage, if in fact, the car doesn't recalibrate for the new sizes - which i don't think it does.

brad