Hello:
As it is mentioned in my sig I have 285/75/16 tires. Good thing is based on my hand calculation, after adjusting the amount of traveled miles by hand and divided to used fuel, my MPG has not changed (or maybe 0.5 MPG loss) which is very good. But the strange part is I have not updated my speedometer for larger tires but when I drive by these side road milage indicator or radars ( lately they have put lots of them in OC) my speedometer is the same as the radar at 40 MPH. any ideas?

Moved to proper section first of all.. Second, your speedo is off. Not by much at 40mph but it is off. I don't know the exact ratio but you will need use a programmer to recalibrate it.

from the factory i see quite a few speedos off. By our dealership their are 2 radars set out so i always see what the trucks read. Usually they read about 2 mph slower then what the radar says. So it's possible your speedo was slightly off and nows it's spot on...

Maybe this will help ya.
http://www.miata.net/garage/tirecalc.html