Sup guys...

Long story short. I just picked up an Extended Warranty and I was reading the list of Exclusions and it says "Any tire that is not recommended by the orginal manufacturer if it creates an odometer/speedometer variance of greater than 4% will not me covered" So my question is will my 20" rims cause a variance of 4 or more??? I've tried to find a chart but no luck... Any help would be apprecaited... Thanks guys...

If the speed is based on how far the car goes per wheel revolution then the following calculation will apply.

The circumference of circle is Pi times the diameter. In other words, the circumference is proportional to the diameter (or radius, since diameter is 2xradius).

So, to calculate the error, if it had 17" diameter, and you now use 20", the ratio of circumference is 20/17, or 1.17. This is a change of 17%.

In fact, even an 18" wheel would chenge the circumference by 5.8%.

Hope this helps.

Tony

Thanks for the info... Well it just looks like I'll just have to take the 20's off for a while.

Hold on, it's not that simple! What matters is the OVERALL diameter of the tire, not the rim size. Example: If you take a set of 16" rims with 235/60R16 tires, and put 18" rims with 235/50R18's, the overall diameter of the tires won't change much at all. The equation for tire diameter is this:

Tire width in mm x (aspect ratio divided by 100) x 2 (for both sides of the tire), divide by 25.4 to get inches, then add rim diameter

Examples:
235 x .60 x 2= 282 mm / 25.4 = 11.10 inches, + 16 inch rim = 27.10 inches
235 x .50 x 2= 235 mm / 25.4 = 9.25 inches, + 18 inch rim = 27.25 inches

So the overall diameter between the 2 examples is .15 inches. Considering that tire wear of 1/4" will change your overall diameter by 1/2 an inch, the difference is insignificant.

So it all depends on what tires were on your 17" rims, and what tires are on your 20"s. :)

If the speed is based on how far the car goes per wheel revolution then the following calculation will apply.

The circumference of circle is Pi times the diameter. In other words, the circumference is proportional to the diameter (or radius, since diameter is 2xradius).

So, to calculate the error, if it had 17" diameter, and you now use 20", the ratio of circumference is 20/17, or 1.17. This is a change of 17%.

In fact, even an 18" wheel would chenge the circumference by 5.8%.

Hope this helps.

Tony

With that theory, you're only taking into account the wheel, not the wheel and tire. If the sidewall size of the 20" is smaller, it may not affect the speedometer.

Use this calculator, and put in the stock tire size, and your current tire size. It will tell you how much the new tires will be off of the stock revolutions per mile.

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

Thanks... The tires I'm running are 245/35/R20 I checked that chart and it should be alright... Thanks again and now I can keep the 20's on.

No problem! :thumbsup: How about posting a pic of your Deville on the 20's?

Thank you for all the replies... Its just that I don't want my Extended Warranty Company saying anything about aftermarket wheels because right now I'm experiencing what I belive are transmission problems and I don't want any complications. If what I veiwed on the chart is correct then I should have no issuses.

As far as pic goes I posted one thread "Show us pics of your Caddy".