Can the speedometer be adjusted after changing to a larger size tires. My GPS says I am doing 5 MPH faster than my speedometer. I figure there has to be some computer adjustment.

 

great question, I know by going to http://www.miata.net/garage/tirecalc.html
I can get the change but it would be nice to adjust permentally.

 

Nope, no adjustment.

 

I think so since there is the optional 18" tire size. Probably programmed by the dealer.


I just checked that. My speedometer is way off then. I put in my current tire size and it said if the speedo shows 60 then I'm really doing 62, but my GPS shows 57!! I would have to go down to a 245/60/16 for it to be accurate. (Wouldn't that size look out of place )


Eric

 

If you were to measure the overall diameter of the 18" tire and rim and compare it to the 16" tire and rim they would be the same diameter.
Thats why the sidewall of the 18" tire is so short and the 16" tire so tall.

 

I had 255 55 18 on it before. At 70 mph my GPS said I was doing 68 mph. New tires are 265 65 18 and at 70 mph my GPS says I am doing 75. Took a trip this past weeknend in a friends Ram Truck. At 70 mph on speedo my GPS marked 70. Thanks for the info. Just thought I would ask if it was possible to adjust.