Horsepower Efficiency Calculator

The old "Horsepower per cubic inch" standard is outdated and useless in figuring out how efficient a motor is,

so you think a Porsche, BMW, Ferrari, or a Mercedes is more efficient just because it's small motor can make as much HP as a bigger motor?

You can pump more fuel in any motor and make more horsepower.

Here's some examples of small cube cars VS big cube cars and their mileage numbers (both in US MPG and European Liters/100km)

Small cubic inch cars
Porsche 977 GT2 (3.6 Liter) makes 530 HP and gets 18.8 (US) MPG (12.5L/100km)
BMW M5 E60 (5 Liters) makes 500 HP and gets 17 MPG (13.835L/100km)
Ferrari 599 GTB makes (5.9 Liter) 620 HP and gets 15 MPG (15.68L/100km)
Mercedes S65 AMG (5.5 Liter) makes 603 HP and gets 17 MPG (13.835L/100km)

Big cubic inch cars
Corvette Z06 (7.0 Liters) makes 505 HP and gets 24 MPG (9.799L/100km)
Corvette ZR1 (6.2 Liters) makes 638 HP and gets 20 MPG (11.76L/100km)
Dodge Viper (8.4 Liters) makes 600 HP and gets 22 MPG (10.69L/100km)

One of the first things I hear when people are confronted with this comparison is

"Well, the American cars run high overdrive ratios to achieve these MPG's"

True, but if they knew anything about camshaft design they would know that a motor designed for 3000, 4000, or 5000 rpms and up is not going to be very efficient running down the highway at 1300 RPM's, and this is due to camshaft duration, as your RPM range goes up your camshaft duration has to get wider to get the air into your motor. When you get wider duration you also get more overlap, or the amount of time that both your intake valves and your exhaust valves are open at the same time (this is what gives an engine that "cool" rough idle).

If you were to try to run this high RPM motor down the highway at 1300 RPM your overlap would result in what is known as "reversion", (some burnt gas going in and out of the intake; and some un-burnt gas going out the exhaust)

The bigger cube motors have smaller duration cams (usually called a torque/mileage cam), so they are very efficient at 1300 RPM's

The bottom line is if all it took for a small cube motor was a tall overdrive ratio to get mpg's equal to the bigger cube motors, don't you think the manufacturers would do it?

The calculator below makes what I call a "Hen number", which makes it easier to compare cars of different HP levels and figure out which one is more efficient.

Simply enter the Car type, horsepower and MPG figures to obtain your HEN number.

If you need HP or MPG Figures for your car you can click on some of the button links below and find most of the numbers you need.

US & Imp

If you have metric mileage values, enter them in the appropriate box and click the conversion button to convert to Miles per Gallon (US)
 US MPG L/100km Imp. MPG km/liter

 Car Type & Model Horsepower (US) Miles per Gallon (US) Horsepower Efficiency Number (HEN) Cars ranked high to low according to HEN number

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