Here's how I do my calculations for ZipZaps, with the 6T pinion gear:

Motor RPM divided by Gear Ratio = Wheel RPM

Wheel RPM times Wheel circumference (which is 4.5 cm)= distance per minute

Distance per minute(in cm) divided by 2.5 = inches per min

Inches per min divided by 12 = feet per min

feet per min times 60 = feet per hour

feet per hour divided by 5,280= actual miles per hour

actual miles per hour times 64= scale MPH

namuna, you need to make sure you zip is up to full speed before it crosses the start line because otherwise that is checking your scale 1/4 mile time, if you do it froma standing start then you are losing scale speed data during acceleration. just a note :D

or u could just spend 5 bucks on this....



http://www.radioshack.com/product.as...5Fid=60%2D7530


and divide the result of mph by 64.