Let see... when my speedo says 32, i'm going 35, and when my speedo says 55, i'm doing 65. Don't know how that works but I followed my girlfriend and told her to do the speed limit and thats how it is. Don't know the point of this post but, eh whatever.

 

the 31" tires will throw the speedo off

i have 30" and they throw it off

also, not all speedos will same the same thing when going to same speed

i know that when i am doing 100 my friends speedo says 105 or so and he drives an accord

 

You can use any number of the available online tire size calculators to find out how far 'off' your speedo will be given the change in tire. My favorite tire-only calculator is Tire Size Calc - Miata Garage (older version if that one doesn't work Old Tire Size Calc - Miata Garage). Most calculators will give you the speedo discrepancy as a percentage of the speedometer reading.