I have the stock 3.55's in my truck with 35" tires but I have 4.10's sitting in the garage that should be installed in the next couple weeks (not lookin forward to tearing down the front end to get at that diff). I'll have to do a before and after MPG comparison to see what the difference is.

 

this might help when figuring the effect when installing a different tire size other than stock

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

 

i get 13-14 with 38's, gears, and a programmer