(motor rpm) x (sprocket ratio) = axle rpm
3800 (max) x 12/60 (est.) = 760 rpm
(if there's a jack shaft just multiply its' ratio)

(axle rpm)x(tire circumference)=inches per min
760 rpm x (15" O.D. tire) = ipm 
760 rpm x (15 x 3.1416) = ipm
760 rpm x 47.124" = 35814" ipm

35814 ipm divided by 12 = 2984.5 ft per min
2984.5 fpm divided by 88 = 33.91 miles per hour

Note: 12" =1' (feet to inches)
88 ft/min. = 1 mph