[uneasyrider]

Here is a calculation you can use.

mph = (rpm x tire dia.)/(gear ratio x 336)

Note: Tire diameter in inches.

Below is a ROUGH calculation you can use to find out the hp it will take to drive your car through the air at a given speed.

hp = mph^3 x frontal area/262500 and add about 30hp for rolling resistance.

Note: mph is to the third power.

Note: Figure the frontal area as the width of the car times the hight of the car in feet.
Note: The constant (262500) factors in the Cd (coefficient of drag) using a Cd of 0.4 as a generic estimate of a typical modern passenger car Cd. If your Cd is known multiply the constant by 0.4 and divide it by the known Cd to get a closer number for the constant.

This one is for 1/4 mile ET

ET = [(weight/hp)^(1/3)] x 5.825 (This formula is good. It gets close.)

Note: It's the quantity of weight devided by hp to the cube root and that quantity is multiplied by 5.825.

or it can be written as

hp = weight/(ET/5.825)^3

This one give you mph in the quarter mile

mph = [(hp/weight)^3] x 234 (This formula is also good.)

Here is an old drag racer's rule of thumb, the best shift point is the peak hp rpm plus 10%. Example: If your car makes peak hp at 5000rpm shift at 5500rpm. That is realy just a good starting point but NOT that EVERY set up works there.