Take my advice and search for posts started by me. There's at least one that applies directly to you and this question.gtjon said:
As for what gears you go with - it's totally your personal preference as well as the application you're putting 'em in. If it's your DD and you do a moderate amount of highway driving, than a 4.30 or even a 4.10 might be too steep for you if you care about gas mileage; unless you want to actually drive the speed limit (or lower) on highways. A little bit of searching will do you a world of good, but I'll spoon feed ya a few things I found when I was doing my gear research. Visit a site like this: http://www.csgnetwork.com/multirpmcalc.html in order to play with your gear ratio and find out what engine speeds (RPM's) you'll be cruising at, at any given speed. That'll give you an indication of how your gas mileage is gonna be. IE You're currently at 2,095 RPM's at 70 MPH with a 25.7" tall wheel/tire combo and a 3.27 gear. All things the same, but with a 4.10 gear, you're looking at 2,627 RPM's ... obviously you're going to be consuming more fuel with a 600 RPM difference.
EDIT: Saved you the "work". Read here for my post: http://www.sn95forums.com/forum/index.php?topic=15224.0
Read here for how to get your total wheel/tire diameter: http://www.miata.net/garage/tirecalc.html
Read here for how to plug in numbers and find out your potential engine speed: http://www.csgnetwork.com/multirpmcalc.html
Read here for what gear ratios your trans has (in stock form): http://www.becontrols.com/ourfaqs/aodefaq_1.htm
