Formulas

The horsepower needed to overcome aerodynamic drag at a given speed is: HP = k*A*Cd*v^3
Where HP is horsepower, k is the density of air, A the frontal (cross-sectional) area, Cd the coefficient of drag, and v the velocity. The density of air depends on conditions but is nominally .08 lb/ft^3.

Aerodynamic drag is calculated as: F = 1/2 CDAV^2
Where: F - Aerodynamic drag force, C - Coefficient of drag, D - Density of air (nominally about 0.08 pounds per cubic foot.. yes I know that's not a technically accurate mass but it saves converting to and then back from metric), A - Frontal area, V - Velocity of object

To estimate engine HP from 1/4 mile run: HP = ((trapspeed/234)^3) * weight

Tire Diameter in inches = ( ( Tire Width * Aspect Ratio ) / 25.4 ) * 2 + Wheel Diameter
Example: 245/40-17 would be ( ( 245 * .40 ) / 25.4 ) * 2 ) + 17 =  24.71653543307

MPH = ( RPM * Tire Diameter ) / ( ( gear ratio * final drive ) * 336 )
Example: ( 8000 * 24.71653543307 ) / ( ( .771 * 4.062 ) * 336 ) =  187.9074535627 MPH
Of course the tires expand at high speeds so this isn't totally accurate.

VERY Rough Rules Of Thumb

Each 10 degree F drop in temperature = 1 % gain in HP
Each 10 lbs. removed from the vehicle = 1 HP for acceleration (top speed is not noticably affected)
Each 10 HP added = 0.1 second in the 1/4 mile

Conversions