[thingstodo]

Going off on a tangent (as I do a lot), and checking various voltages ...

Raising the battery voltage to 72V with Lithium Ion Thundersky batteries (higher peak current, lower weight) drops the weight and increases the available current. Third gear appears to be the best acceleration, still at rated speed, and close to highway speed. WAY TOO FAST FOR A PARKING LOT!!!
20 mph 48V 70 amps 3600 rpm first gear
35 mph 72V 100 amps 3600 rpm second gear
50 mph 72V 150 amps 3500 rpm third gear
47 mph 60V 150 amps 2500 rpm fourth gear
45 mph 48V 150 amps 1900 rpm fifth gear

Raising the battery voltage to 96V with Thundersky's. Third gear still appears to be the best acceleration, still at rated speed, and closer to highway speed.
20 mph 48V 70 amps 3600 rpm first gear
35 mph 72V 100 amps 3600 rpm second gear
50 mph 74V 150 amps 3500 rpm third gear
55 mph 72V 150 amps 3000 rpm fourth gear
55 mph 72V 150 amps 2300 rpm fifth gear

Raising the battery voltage to 144V with Thundersky's. Fourth gear appears to be the best top end ... but acceleration will suffer.
20 mph 48V 70 amps 3600 rpm first gear
35 mph 72V 100 amps 3600 rpm second gear
50 mph 74V 150 amps 3500 rpm third gear
65 mph 90V 150 amps 3400 rpm fourth gear
65 mph 78V 150 amps 2800 rpm fifth gear

OK - that's enough investigation into weights, batteries, and voltages