[starscream]

OK here's some calculations for ya.
Assuming you have a center diff/slipper and 1/8th F/R diffs
If you have a transmission or other gear reduction, the gear ratio will need to be re-calculated.
Here's a gear ratio break down
Center diff/slipper F/R diffs
14t pinion 12t pinion
51t spur 40t spur
51/14=3.64 40/12=3.33

So 3.64*3.33 gives you a final gear raitio of 12.14 to 1

Now for the other variables,
4s lipo = 14.8v
7xl kv = 2382
At this voltage the motor can spin at 14.8*2382 = 35253 RPM

So divide your motor's max RPM by your gear ratio to get the wheel's max RPM;
35253/12.14 = 2903RPM

The 40 series tries are around 18inches around (circumference)

So 2903*18=52254 inches per minute
Which is 4354.5 feet per minute
Which is 261270 feet per hour
Which is 49.48 mph

The 12t pinion looks like it will push your truck around 40mph, and the 10t looks like its around 35mph.

Of coarse the size of the truck (weight etc) play a role in the performace and speed the motor can produce.

I'd start with the 10t and work your way up. I would error on the side of more gear reduction than less, especially starting out. You can always gear higher :)

I listen to Dafni about motor temps. Heat gun's are definetly worth thier weight...

Oh ya, a 50 to 1 ratio would put you at 12mph ;)

Cheers