I got ticketed for going 79 in a 65 today on my way down to San Jose this morning. According to my speedometer i was doing 70MPH. But the CHP said i was doing 79MPH. I didn't even ask for the radar read out cause i was running late and wasn't in the mood for BS so i said whatever ticket me and let me be on my way. So my question is does wheels size affect the speedometer reading on your car? I have 20's on my car. My first ticket in 7 years . Personally i think they needed to make their bs quota and was looking for every reason they can to ticket people.

 

when I was running 20s, 245/35/20 front and 275/30/20 my speedo was fine, the rolling diameter was the same as the stock 17 tires

 

Yes, tire size does affect your speedo.

Check out this tire size calculator: http://www.miata.net/garage/tirecalc.html

It will let you plug in various sizes and tell you how far off your speedo will be. Just remember that your stock tire size is 26.7 inches for the coupe, or 26.3 for the sedan.

 

As long as you get the proper setup, you should be within 1MPH of of the OEM wheel setup.

You should stay as close as possible to the OEM tire diameter.

 

If your tire diameter is larger than the stock diameter.

Just remember the faster you go, the difference between what your speedo says and what your actual speed is increases.

 

Thanks guys. I'm gonna swap out the 20's for the stockies for awhile.
tired of driving on 20's anyways and the wheels do draw a lot of attention to the car as well. My car is already red and the wheels are black center with red lip.

 

yea that is all true about a slight overal diameter difference will make good bit of difference at higher speeds but also speedometers are not perfect some infiniti speedos have been know to be -+5mph on stock wheels get a gps if you really want to know how much it is off at a certain speed