I use this calculator to find needed Watts at desired cruising speed.

http://ecomodder.com/forum/tool-aero-rolling-resistance.php
It is better to make everything a bit worse than reality when using a calc like this than be optimistic and melt your motor in the real world. So I up rolling resistance, weight, and aero drag a bit, just to be sure.

Obviously, you divide by system voltage to get steady amp input. The motor you choose had better be rated for at least an hour at that amp rating.

Conversely, you can take the 1hr rating of your motor and multiply it by your voltage to see the max watts at that voltage. If you come up short, you need to increase voltage if you plan to keep the motor.

I have a vw Bug chassis with a Jaguar body that is light, but not aerodynamic.

The ADC 8" has a continuous rating of 180amps. The system voltage is 96v. That equals ~17KW (23hp). Worst case scenario the calc says I can go 65mph steady. Best guess numbers put it closer to 70mph. The commissioner of the car wanted a cruising speed of 60mph, so I have a bit to spare. A good thing. If he wanted to cruise on the interstate at 80mph, then I would need to up the voltage to 144v. The current build should do 80mph, just not for an hour.

Forced cooling will allow you to get a bit more from the motor, so if you are encountering heat issues, more cooling and higher voltage are the answers.

Also, 80+% of my energy is going to move air. So aero mods can go a LONG way toward increasing top speed, reducing amp flow, and increasing range. Reducing the aero drag from .5 to .3 would suddenly allow steady cruising over 75mph with the same system, and a 33% drop in the energy required to go 65mph. That means 33% MORE RANGE! So my 100 mile range would suddenly increase to 133 miles.

This list shows the best donor vehicles by lowest cdx:

http://en.wikipedia.org/wiki/Automotive_aerodynamics
It is an incomplete list and they omit many such as the Eagle Talon which is just after the 2002 Acura NSX at 5.85.

You can see that a Honda CRX is the best low-dollar vehicle to convert. 88-92 years have best aero, but 86/87 is not much worse and a bit lighter. There are only a couple of crx conversions on the evalbum, and only one of those is lifepo4, so it is a really under-used chassis considering it's potential efficiency.

Ecomodder.com has shown how to radically reduce aero drag. I am surprised the EV crowd is a little slow to jump on the band-wagon since there it would increase EV viability greatly.