BrianG's speed calculator is a great tool for this. A 2000kv motor on 4s is going to make it hard to gear for that kind of speed. 58/24 gearing will get you about 45MPH but I don't know if that gearing will actually fit and work. Generally on 4s a top speed of 40MPH is your best bet, after that your going to be stressing your batteries and esc and maybe the motor. How close is your 54/18 to meshing? I would try using the pinion you have and get a 56 or 58 spur and see how that works for you, spurs are cheap, pinions are not. Good luck getting it figured out.
Just my 2 cents here now. Try gearing for about 35 and see how you like that. Personally I have always geared my monster trucks down and had more fun with them. Currently I have a Savage Flux, came geared 44/20, I'm now running 49/17 and have a 52 spur on the way to run 52/15 and possibly drop down to 52/12
Speed calculator
http://scriptasylum.com/rc_speed/index.html