Minimum voltage to achieve 60 mph?

[Stew-2]

Lets say for the sake of this question, the car weighs around 3200lb ready to drive.

First question: What has been peoples experience as far as a minimum voltage to maintain 60 mph? 96v, 120v, etc. Acceleration isn't so important.

Second question: Would it matter if it was an 8" vs 9" motor?

[madderscience]

Totally off-the-cuff, I'd say 120v with a 9" motor, maybe 96v or so with the 8".   It depends on other factors like gear ratio and the aerodynamic properties and weight of the vehicle as well but 96v is generally considered the minimum for a freeway capable full-size vehicle conversion.   

My MR2 could barely hold 60mph when it had a 102v configuration with a 9" motor and about 4.7:1 final drive ratio and 23" diameter tires.   Once upgraded to 126v but otherwise the same setup, it can now hit 70mph without much trouble.
 
Smaller diameter motors will tend to rev higher with the same input voltage but with less torque.  Larger motors will be more torquey but take more volts to get them spinning.  This should not be taken to mean one type is inherently better/worse or more/less efficient than the other, they just factor their output power differently.  (speed * torque == horsepower)  scale one up and the other down proportionally and the output power does not change.