So if I am going 60mph with the old tires it will show I am going 62mph with the new tires? or is it the other way around???

 

If the speedo says your doin 60, your really doin 62. I don't recall it being that much tho, maybe 61.2-4.........

 

If you upsized from 245/50-16 to 255/50-16 yeah, speedo will be low by about .9 mph.

Here's an easy way to compare:

http://www.miata.net/garage/tirecalc.html

Speedos aren't really that accurate, you know. 2 mph accuracy at 60 is probably about factory specs. If anything they will read high not low, so the 255's might even correct your speedo!

FWIW speedo accuracy is easy to check:

On a highway with mile markers, plug in the cruise at some indicated speed and accurately time a mile with a stopwatch.

3600/seconds = mph

so 60 mph is 60 secs, 70 mph is 51.2, 130 mph is 27.7.

If you're doing it at 130, please have a passenger do the timing.