Just to get an idea of the difference between a GPS and Traxxas Link, this is what I get on my
2WD slash stock out of the box set up using a 7cell NiMH 5000 mAh battery at starting charge of 9V (not fully charged). Traxxas link reads 36 mph, iPhone GPS reads 31 mph over an average of 4-100 yard speed runs.
ERBE using 18/54 gearing, road rage taped but not balanced tires on 1/2 offset desperados, 3s at starting voltage of 24.6v by traxxas link (12.22v + 12.17=24.39v measured on voltmeter). Traxxas link reads 60 mph, iPhone GPS reads 46.5mph over 6-100 yard speed runs.