My 5-spd will barely hold 50 mph in 5th gear, unless I'm going down a hill! I've also determined that the 5-spd in my 2001 is an NV3550, not an AX-15. Anyone know the gear ratios in that tranny.
In the end, though, the math is really pretty simple for calculating the gears you need. Whatever percentage you increase the tire size by, you need to reduce the gear ratio by at least that same percentage. So, if you go from stock tires that are roughly 28" in diameter to tires that are 31" in diameter then it is... 31-28 = 3; 3/28 = 0.107; assuming stock gears of 3.55, 1.107*3.55 = 3.93; nobody makes 3.93 gears so you have to decide between 3.73 and 4.10.
If you live in Iowa (or someplace equally flat) and do a lot of highway driving then you might want to go with 3.73. Most likely, though, you'd be happiest with the 4.10 gears. This is the subjective part of gear changes.
In any case, the math is the same, you just change the numbers for tire sizes and/or final drive ratio to match whatever yours is. If you want to know how high your engine will rev after a gear change it's the same kind of math, only a little easier. Let's say it runs at 2100 rpm in overdrive at 70 mph with stock gears. Again, assume stock gears are 3.55 and you're thinking of going to 4.56. So, 4.56/3.55=1.28; 2100*1.28=2688; this tells you that with the new gears and your current tires you'd be running near 2700 rpms at 70 mph in overdrive.
2700 rpms is a little high and would probably lower your gas mileage some, but it is far from "too high," you'd have no problem running that all day long, and the engine would pull right up to that speed with no trouble at all.
The math is simple, though, and you apply the same basic calculations to any speed/gear combination to figure out the results.
Good luck!
