Well, the calculations aren't all that tricky for best performance. F=M*A covers most of it. I don't mind giving it a try, though I could be completely off.
lessee, we have to know :
1. The torque of the motor @ 72 volts
2. The rpm range of the motor w/72 volts available.
3. The gear ratios of the transmission you are interested in
4. The overall gear ratio and tire size
5. the weight of the vehicle.
Given the flat torque curve, it should accelerate at a fairly even rate, excluding drag.
But I don't know enough to talk about it in general terms, I need a specific example.
So take the 9" warp:
Shock EV | NetGain WarP 9 Electric Motor- 9.25” diameter, series wound DC motor
- Weight, approx. 156 pounds
- 32.3 HP (72 Volts, 335 Amps
- 70 Ft. pounds torque*
- 5,500 RPM's
You have guessed 1200lbs
And lets assume your batteries are "standard" T-105s
Golf Cart Batteries and are in good shape to deliver 335 amps.
and lets assume the rear tire is 24" and revolves 840 revs/mile
You stated an overall ratio for 1st gear, 400:1 (???)
So lets run the numbers for that,
so basically the torque on the rear wheel in first is 70*400 or 28000 foot lbs. Since the radius of the tire is 1 foot, you basically have 28000 pounds of thrust. But it is turning 400 times slower than the motor too, so when the motor redlines in first at 5500 rpm, your rear wheel is going 13.75 rpm, or about 1 mph, and now it is time to shift to second. I don't think most hyubasa owners shift at 1mph
On the other hand, if you want top speed to be 70mph @ 5500 rpm, your top overall ratio needs to be 5.6 to 1. And the tire will be pushing the bike with 392 pounds of force (70 * 5.6), which sounds reasonable to me.
So lets start there and add some gears (I'm gonna ignore aerodynamic load for now)
convert to metric, 392 lbs = 1744 newtons, 1500lbs=6672newtons, 70mph=31.3 meters/second
With direct drive, 1744/6672 = .26 = A , so 31.3/.26 = 120 seconds to 70mph
lets add a 11.2 ratio first (overall) and a 5.6 2nd. So acceleration has doubled to 35 mph. This first gear should be our biggest time gain, the rest will be diminishing returns. So off the top of my head it looks like 90 seconds to 70mph. 4 gears might get you down to 75 seconds.
So if I were a gambling man, I'd say the limit, assuming infinite gears and zero shift time, would be 60 seconds to get to 70mph.
Not exactly "blazing", and I could be completely off the numbers here, but I don't think a close ratio box is going to help matters.