The speed calc does simple equations, so its only approx. It doesn't do anything with weight, aero, loses due to friction, etc.
Probably .5". I haven't been using much as my strapped ProLine Badlands (gorrilla tape) seem to not balloon hardly at all.
What I meant was that if you setup a standard Flux with 5S, 2200kv motor, 44t spur, wheels (I used Dirt Bonz), 0" balloon and 25t pinion you get just ~60mph. If you swap out the 2200kv for say a 1800kv motor, you'd need a 32t pinion (which I'm not sure they make) to get the same ~60mph.
As far as new gearbox ratio, you'd just enter it manually.
Well tower has them, but not til middle of the month...
http://www3.towerhobbies.com/cgi-bin...?&I=LXZGP9&P=7