Remember that I am being very conservative with my numbers

Well if I got this right, your car will weigh 1100 lbs (w/o batteries and motor).

Our motor weighs about 200 lbs, but it’s huge (your probably not going to use something like that). So lets say your motor weighs 150 lbs (that is still a heavy motor). Now for the batteries, just to get 72v I would recommend six 12v batteries. which weigh any where from 60-85 lbs. I think a 75 lb battery is a good guess, and six of those is 450 lbs. So the total weight should be somewhere around 1,700 lbs.

the equation for aerodynamic drag is Fd = (Cd*A*V^2)/391

Fd is the force due to aerodynamic drag

Cd is the coefficient of drag

A is the area of the front of the car

V is the velocity of the car

the Cd has to be somewhere around .3, but I think it’s better to over estimate. So I’ll use .35.

Fd = [(.35)*(90 sq ins)*(45mph)^2]/391 = 32.628 lbs

multiply this by 150% (so we have a margin for error) = 48.9 lbs

which is equivalent to 50 lbs

Next the equation for rolling resistance is Fr = Cr*W*cos(theta)

Fr is the Force due to rolling resistance

Cr is the rolling resistance factor

Cr = 0.015 on a hard surface (like concrete, I think you want this one)

Cr = 0.08 on a medium-hard surface

Cr = 0.3 on a soft surface (like sand)

W is weight in lbs (as opposed to Newton’s, Ha)

cos is a basic trig function (won’t go into details)

And theta is a Greek symbol used to represent angles (in this case the angle of your tires, lets assume your going strait, so theta = 0. And the cos(0) = 1)

Fd = (0.015)*(1,700 lbs)*cos(0) = 25.5 lbs

multiplied by 150% = 38.25 lbs

which is equivalent to 40 lbs

So the total drag on the engine should be about 90 lbs, but lets make it easy and say 100 lbs.

The equation for horsepower is HP = F*V/375

F is force in pounds (by now you know F = force)

V is velocity in mph

HP = (100 lbs)*(45 mph)/375 = 12 hp

The equation for Watts is W = HP*745.7

W is Watts

HP is horsepower

W = (12 hp)*745.7 = 8,948 Watts

To get Amps we use I = W/V

I = current in amps

W = Watts

V = volts

I = (8,948)/(72v)

I = 124 amps