If you want to estimate the top speed of your car, use this formula:

First measure the Diameter of the Tire.
Then multiply the Tire Diameter by Pi (3.14159).
Let this number be "y"
Calculate your Final Drive Ratio ( spur divided by pinion times the internal gear ratio)

Now calculate the RPM at the tire. Divide the maximum motor RPM by the FDR.
Let this number be "z"

I get 99mph. About what the venom speed meter says under no load. (just holding it in the air and giving it full throttle). Your formula is accurate, I guess.

Understand that under no circumstance is the motor EVER going to reach its Max RPM while driving. Unless you were on the moon. If you want to really figure how fast your car is going approximately, take about 7-8000 rpm off the max RPM and calculate with that.