Well I do know my speedometer said more than 35 I just don't know exactly what it said because I was a little distracted by the cop.

 

In my opinion, the easiest solution is a GPS. They aren't affected by tire size.

 

My speedo was 5mph slow with stock tires. I agree with the GPS, I don't ever look at my speedo for the speed anymore.

 

First of all if you switch to 31s from 235/75/15s then the speedo will read 7% slow, and to all the ppl that said there's is 8 off or 10 off of whatever, you speedo can not be off by the same speed all the time like that it is a percentage. if you say it 8 off then it would have to be 8 off at 0mph and thats not possible. at 7% slow you would be doing 65 when your speedo said 60, and when you got pulled over for 50 in a 35, you speedo should have said 47mph

 

and when you go to check your gas mileage you have to mutiply it by 1.07 for it to be accurate

 

How did you calculate that? It would nice to know my actual MPG. I have 33s.

 

people have said that the odometer is off as well now. Is that in my favor (says less miles than actual) or against me (says more miles than there actually are)?

 

it will show less miles than actual

 

Tire size calculator - Miata.net
Put in your stock size and then your new size and it'll give you how much of a difference there will be.

 

for 33s your speedo and odometer will be off by 14% to calculate for gas mileage as usual then mutiply your answer by 1.14 you get your actual.