rustler/stampede question

speed/runtime/cells...... uh, i dunno..... like i said, im a newbie sorry....

im running 3000 mah ni-mh right now and am only getting about 10 min. of good runtime on stock stinger motor, no local tracks so this is all just messing around in yard and ditches, etc.... still dont seem very fast though, maybe 15 - 20 mph at most, dont have a radar gun, couldnt tell you for sure.

also, would changing gears in tranny to steel help and what sizes or where to buy?, completely new, i know nothing!!!!!!
just got out of motocross so thats all i know