Maximum speed calculations... [TANKS]

[himself-at-adamwells.net]

I'm just hoping somebody can verify my calculations...

I'll be using 12v DeWalt Drill motors and gear boxes which are rated at 450 no 
load RPM and a maximum of 350 in-lbf in their lowest settings. I plan on mating 
12 pitch 15t miter gears to 12 pitch 18t miter gears for a further 1.2 
reduction that should give me 375 RPM and 420 in-lbf. The drive sprocket will 
be a 28t sprocket for #40 (.5" pitch) attachment chain, which should give me ~5 
mph theoretical top speed (with actual speed under load being lower... I'm 
shooting for an actual practical top speed of around 4.25 mph). Am I correct?

Adam