18:54 or 19:54 would be a good starting point. That's 36-38mph on 4S and 45-48mph on 5S.
I came to the same conclusion of 18-54 as i have the 18T pinion (also have 22 and 24 but those are pretty big) and nearly all of the available spurs minus a couple. I just wanted to check and make sure that this wouldn't be too much for the motor/esc and batts.
Toys:
Slash with VXL
E-revo with MMM/Teckno Nue 1515/1yf running 4s and 5s