Thanks.
I've tried to calculate the ratio in city traffic(pretty inaccurate yes, but I didn't have the time to take a ride on the highway).
Just edited the post since I was wrong....
My tyres are Michelin, 205/60R-15, worn. That makes them about 0.5in less in diameter than standard.
According to this source:
http://www.miata.net/garage/tirecalc.html
revs/mile ratio for a 195/65R-15 is 807, for a 205/60R-15 is 817, and for a 205/55R-15(roughly similar to a worn 205/60R-15) is 845.
Tried the formula - it works... I can say I don't have a 3.92:1, but the difference between 3.07 and 3.27 is too small to tell with accuracy
~Nautilus