Can you calculate the difference in the speedometer reading after increasing your tire size? I just went from the stock STX 17" Continentals to 33's and am curious if there is a way to tell how far off my speedo is now (without a radar gun). Thanks.

 

Question ... was the speedo correct before the tire change?
A GPS works great! Or you can estimate with:
Speedo & Tire Calculator

 

Yep, the speedo was correct before the change.

Thanks for that site. That answered my question.

 

I just had my wife drive next to me with windows down to tell me her speed or with our Nextel's.

 

Try this:

http://cat.cc/tiresize.htm

 

Go to Discount tire website and use their calculator.

 

For a quick calculation, measure the circumference of the old tires vs the new tires as a percentage and use that number for the speedo correction factor. If the old tires measured 100" and the new tires are 110" then you would have a 10% correction factor so if the speedometer showed 60mph you would actually be going 66 with the bigger tires.