Yep, I'm still selling my DVD. (All proceeds go to help future projects!)
It's available at:
Electric Car Instructional DVDs
Getting to 50 mph is a bit much to ask on a 72V system. I recently heard of a "rule of thumb" that you can ball-park top speed by system voltage equaling speed in kilometers per hour. Top speed in my Electro-Metro at 72 volts is 45 mph. If you convert system voltage (72V) as KPH to MPH, you get 72K = 44.739 MPH. Wow - not bad for a "rule of thumb!"
AC motors are great, but they tend to use higher system voltage ( more batteries, more weight, more expensive charger) and are more difficult to find at an appropriate size/weight affordably, compared to a DC motor.
In terms of horsepower of a motor - it's "ooomf!" times speed.
On a series-wound DC motor, the more voltage your give it, the faster it spins. The more amperage that gets draw through it (because of being at a stop, accelerating, gear selection, going uphill, etc) the more oooomf! it has. (torque)
Speed x torque = horsepower
Voltage (motor spin speed) x amperage (torque) = Watts - power required.
There's roughly 746 watts in one horsepower. So, it's pretty easy math to figure out how many horsepower a motor can produce. (And keep in mind that it's INSTANT power! None of this having to rev an engine first!)
The limitation is how much amperage a motor can draw. A small motor can't, won't, and shouldn't draw a high amount of power. A beefier motor can.
And since system voltage is part of the equation that makes horsepower - more horsepower can be created by increasing the voltage. Or, all other things being equal, you can make the same amount of power (watts) by using higher voltage, but less amps. The batteries appreciate you using less amps! That's for a number of reasons, including the Puekert Effect, C-rating, etc.
Most DC motors can spin pretty fast, just don't go around reving them with no load, because then then can spin so fast you can't imagine.
Watts per mile (electric "fuel-economy" is probably going to be at whichever gear will LOWER the amperage draw and still allow you to drive the speed you would like.
In my experience, it tended to be in one gear LOWER than you would have used with gasoline. The electric motor spins faster, but it's not working as hard, so it lowers your amperage, which improves how much power you are using to drive the car.
Higher voltages give higher top speeds and better acceleration (compared to the same amperage at a lower voltage) and generally gives the car better range and makes it more fun to drive.
Geo Metros are a bit limiting in how many batteries you can put in them. I've put up to 144V in mine. You need improved suspension at that point. (Mine has truck springs in the back.)
The Solectria Force was a factory converted Geo Metro sedan that used 13 Group 27 gel cell lead-acid batteries. Some of the batteries were up front, and the bulk of them were under a false floor in the trunk area. Those were AC motor cars with regenerative braking and even air-conditioning!
The wildest thing I have ever seen done with an electric Geo Metro is this one:
Dave Cloud's Rally Car
but if you are doing something like that, it's BEYOND what my experience can help you with!