Circuits are derated at 80% with continuous loads so a 100 amp circuit will supply 80 amps (slightly exceeding the 72 amp onboard charger) while the 80 amp circuit will supply 64 amps (slightly below the 72 amp onboard charger).
64 / 72 = 0.88
0.88 * 45 mph = 39.6 (or ~40 mph)