I checked it against this program:
http://www.waits.net/public/rcgears/
and found that for the setup I will be using in my Rustler a difference of about 0.5mph came out between the two programs. I don't know which one is more accurate, but I am just pointing that out.
SH Z-Car, Custom Crawler, 8s Savage,
12s XTM XLB 1/7 buggy,
4wd 4-link rear/IFS Pro4 truck,
Custom Hyper 10 Short Course,
Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz