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

Volume
1 Imperial Gallon =	4.5461 liters (l)
1 liter (l) =	0.2200 Imperial Gallons
1 US Gallon =	3.7854 liters (l)
1 liter (l) =	0.2642 US gallons
1 Imperial Gallon =	1.2009 US Gallons
1 US Gallon =	0.8327 Imperial Gallons
1 liter =	61 Cubic Inches (ci) [approximate]
Distance
1 mile (mi) =	1.6093 kilometers (km)
1 kilometer (km) =	0.6214 miles (mi)
Fuel Consumption
1 Mile/Imp. Gallon =	0.354 kilometers/liter (km/l)
1 kilometer/liter (km/l) =	2.825 miles/Imp. Gallon
1 mile/US gallon (mpg) =	0.425 kilometers/liter (km/l)
1 kilometer/liter (km/l) =	2.352 miles/US gallon (mpg)
Mass
1 kilogram (kg) =	2.2046  pounds (lbs)
1 pound (lb) =	0.4536 kilograms (kg)
1 gram (g) =	0.0353 ounces (oz)
1 ounce (oz) =	28.35 grams (g)
Pressure
1 pound/square inch (PSI) =	0.0689476 bars
1 bar =	14.5037681 pounds/square inch (PSI)
Power
1 PS =	0.98632 HP
1 HP =	1.01387 PS
1 kgm =	7.23 lb. ft.
1 lb. ft. =	0.14 kgm

Reference