Coast Down Power Test

This tool calculates the amount of power needed to move a car at a steady speed as measured in MPG. This tool is useful for measuring the total power load and thus MPG capability of the car.

Power produced by the engine is used for/consumed by:

  1. Accelerating
  2. Going up hills
  3. Aerodynamic drag
  4. Overcoming mechanical friction including:
    • Drag due to toe-in. (Learn more about toe-in)
    • Wheel bearings
    • Tire rolling friction
    • Brake drag (if any)
    • Drivetrain friction

Many cars are capable of getting far better MPG! This tool is intended to help identify problems which cause excessive drag and friction. This tool uses a measurement of how long it takes to coast from 70 mph to 60mph. This duration (in seconds) is used to calculate the total power consumed by aerodynamic drag and mechanical friction.

Here's how to perform the test:

  1. Find a level strech of highway.
  2. Accelerate to just above 70 mph then put the transmission in neutral.
  3. Time how long it takes for the speed to decrease from 70 mph to 60 mph.
  4. Repeat the test in the opposite direction and average the results. This will serve to cancel out any effects due to wind or slope.

Weight of the car, including the driver and any passengers (lbs)
Starting Velocity : Vstart (mph)
Ending Velocity : Vend (mph)
Time to get from Vstart to Vend (seconds)

Horse Power consumed by your car is 23.652
Miles Per Gallon = 30.739

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