Truthfully, probably not all that much. But
who are thinking of designing or building a car for high mileage or if
you are just interested in how much power it takes to drive your car
might be curious.
This article wiil show you how to measure the power to drive a car at a
steady speed down the road. Actually the technique/formula can also
be used to measure the Hp applied to the wheels while
What do you need to take measurement?
To measure how much
Hp it takes to drive your car at a steady speed you need nothing more
stop watch, an accurate speedometer and knowing the weight of your car!
I think a lot of people will be surprised how little Hp it really
And also how much overpowered most cars are to have good
General idea of how to take measurement
November 1970 issue of Road & Track magazine had an article
presented several formulas and descriptions of how to measure Hp at the
drive wheels in each gear and other interesting data. I'll just present
the method and formula for the steady state Hp to move the car.
what you do to take the measurement is drive your car at a constant
speed on a level road at a speed faster (the article suggested 10 MPH
faster) than the speed you want the measurement to represent. You then
put the car in neutral and start the stop watch at the same time. You
stop the watch when the car drifts 10 MPH slower than the speed you
want to know the Hp needed to drive the car at a constant speed. The
results of the various numbers/calculations will give the Hp at
the average speed of the two readings.
Taking the measurement
To find out
how much Hp your car needs to drive at a constant 60 MPH for instance,
find a level
road with no wind blowing, no other cars around and drive at a steady
70 MPH. When you are ready to take the measurement, push the clutch in
and start the stop watch. When the car drifts down in speed to 50 MPH
stop the stop watch and record the time it took.
You should take the
measurements several times in opposite directions to cancel out the
affects of a non-level road, slight breeze etc. Do not do the
test following another car or truck. You want the air your car is
traveling through to be still.
Here's the formula you need to punch the numbers into
Hp = .0001216 * (W+R) * v * (V'/T')
a constant derived from the acceleration of an object when acted upon
by a steady force (based on Newtons laws) and the conversion of ft/sec
Hp = power required to move car at a constant speed.
W = weight of car in lbs.
R = a term known as "effective rotational weight" of the
wheels, tires etc (use 100 lbs)
v = average speed, in this example 60
V' = is the speed difference from the start & end of the test
(70 - 50 = 20)
T' = time in seconds to drift from the start to end speed of the test.
Here's an example of the numbers taken with my Honda CRX
Hp = .0001216 * (1800 + 100) * 60 * (20 / 23) =
12 horsepower at the drive wheels to move the car down the road at 60
The max Hp that my car has (60 big ones!) is used for acceleration and
better performance etc. Don't
rush out and put a lawn mower engine in your car unless you like 0-60
MPH to be, "Why yes it will do 60 MPH!" ............ (eventually).
Some day I need to take the same measurement on my Honda
The same formula can be used to measure how much Hp your engine
delivers to the wheels while accelerating.
Something to consider
Many people will say that the weight of the car
not affect, to a large extent, the amount of wheel Hp (WHp) to maintain
a vehicle at a constant speed.
We all realize that it takes
more power to accelerate a heavy car as compared to a light car. While
you accelerated the car to the speed you want to drive at you applied
force to the mass of the car. The mass had kinetic energy added to it.
you do the above measurement, the car is slowing down due to
aerodynamic drag, friction of the wheels etc. The kinetic
expended to keep the car moving. If the car had zero weight (and hence
zero kinetic energy) aerodynamic drag would stop the car very quickly
because there would be no kinetic energy to counteract against the
aerodynamic drag etc.
If you don't believe this, toss a small piece of cardboard out of the
car window into the air flowing by while driving 60 MPH and see how
quickly it slows down in comparison to your car if you had pushed the
clutch in at the same time when you let the paper go.
maintain a constant speed you have to apply some amount of constant
force to the mass of the car to counteract the force being applied by
the aerodynamic and other drag forces that are trying to reduce the
kinetic energy of the mass of the car. The heavier the car, the more
force (Hp) you have to apply.