First you need to collect a few pieces of information. Go here and plug your tire size into the calculator http://www.miata.net/garage/tirecalc.html and it'll give you revolutions per mile as one parameter. My 285/70-17s revolve 617 times per mile. Then you also need to know your transmission gears. My truck has the 545RFE and the ratios are listed up right on wikipedia as well as other places (2nd is 1.67:1, 3rd is 1:1, 4th is 0.75:1, 5th is 0.67:1).
Once you have that isn't just punching up the numbers, if you make a table in excel you can compare what speed you'd be in different gears for a give RPM or vice versa.
MPH = (RPM x 60 minutes per hour) / (REV per MI x Rear axle ratio x Trans gear ratio)
So in my truck with 3.73 rear gears you can look at what 2300rpm gets you in terms of speed in terms of the various trans gear you're in:
(2300 x 60)/(617 x 3.73 x 1.67) = 35.9mph in second gear
(2300 x 60)/(617 x 3.73 x 1) = 60mph in third gear
(2300 x 60)/(617 x 3.73 x 0.75) = 80mph in 4th gear
(2300 x 60)/(617 x 3.73 x 0.67) = 89.5mph in 5th gear
If I went to 4.11s in the same scenario
(2300 x 60)/(617 x 4.11 x 1.67) = 32.6mph in second gear
(2300 x 60)/(617 x 4.11 x 1) = 54.4mph in third gear
(2300 x 60)/(617 x 4.11 x 0.75) = 72.6mph in 4th gear
(2300 x 60)/(617 x 4.11 x 0.67) = 81.2mph in 5th gear
On my truck being the gas engine it is, I really need to be 2000rpm or better to make torque needed to pull up grades (diesels are a different story). When you make a table of it all there is a nice gap in the 2nd to 3rd shift. Second needs to pull into the 3500-4000rpm range to down shift 3rd into the 2100-2300 range to be moving around 55-60mph, when on the expressway I can quickly spool up 2nd and drop into third and keep pulling in that gear. However if I am on a slower highway in which I'd need to hold something like 50mph up a grade 3rd gear is too low of an RPM and instead I'd need to sit in 2nd gear at 3200rpm. If I switch gears to 4.10s I can do that at 2150 and should still be in a position to hold the gear. 4.56s would be awesome in that I'd be at 2350rpm which means I'd have even better performance in the 40-60mph range but would take a beating on the higher speeds (as mentioned above 80mph would be 2500rpm or so verse about 2250 with 4.11s).
As mentioned this is all mainly because I'm not running the smaller tires the truck was designed for. Not that I'm running huge tires but it does make a difference when you add up the tires/camper weight/steeper grades. I could live with/deal with the 3.73s but its not ideal and I think it'll be easier on the truck if I eventually go over to 4.11s. I'm not rush you the door to do so though.
