I answered this question yesterday but it disappeared. Not sure if it's a problem with the site or not...

I just swapped the stock 16 tooth bell on my Tyrannosaurus to a 13 tooth from an HPI Savage, in anticipation of the direct drive transmission that's on it's way to me. $16 at my LHS for the clutch bell. If you're staying with the stock transmission, you could try 14, or 15 teeth.

IIRC from what I looked at the other day, the overall ratio between crankshaft speed and the stock transmission output shaft speed is 4.75:1 The 15 tooth bell would up that to 5.07:1, the 14 tooth bell would give you 5.5:1. The 13T and DD combo is about 4.38:1

For a LITTLE more acceleration I'd go with the 15 so that you don't kill the top end.

Supposedly the 13t bell and the direct drive transmission gives you better overall speed with an increase in acceleration due to reduced friction. That would be another $20, though, but is supposed to be worth it.

Edit - I had my numbers totally screwed up so I adjusted them.