It depends on where/how you drive it. If you race on a track with long straights, then you can gear taller to take advantage of the speed. If you race on a track with short straights and very technical section with quick jumps, gear lower to use more of an RPM range with the engine.
For general purpose, I find that 15/38 is a decent combo for bashing. Fast enough to get you into trouble, but enough grunt to pull a wheelie. Thats what I ran when I had a 18TM. Now I run a BB and I should really gear it taller, but I only went to 15/36... lots of easy wheelies, but it still goes really fast.
The only reason I stated "race" first, is because of your tires/setup. It appears to be setup more for racing than bashing. I'm just a basher only, so my tastes would probably differ greatly from a racer.