RC Monster Speed Estimate Calculator

[Patrick]

With the rpm the motor is doing you'll need about 14:1 final drive ratio to do 40mph (maybe less, depending on voltage drop), and the setup you posted has about a 31:1 ratio. Just changing the diffs to buggy ratio ones gets you to 24.1:1, so that's the first thing that need's to be changed.
Then go to around 66/24 like jhautz said to get 15.66:1 and around 35mph.
If you already have the truggy ratio diffs (I hope you don't) then you need 51/24 to get the same ratio, and that would be about the fastest you could gear it (that I know of).