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Speed to get maximum fuel efficiency
What speed should I drive to get maximum fuel efficiency?
.
In general, smaller, lighter, more aerodynamic cars
will get their best mileage at higher speeds. Bigger, heavier, less aerodynamic
vehicles will get their best mileage at lower speeds. If you drive your car in
the "sweet spot" you will get the best possible mileage for that car.
If you go faster or slower, the mileage will get worse, but the closer you
drive to the sweet spot the better mileage you will get. For most cars, the "sweet spot" on the
speedometer is in the range of 40-60 mph. Cars with a higher road load will
reach the sweet spot at a lower speed.
HowStuffWork
In
general, smaller, lighter, more aerodynamic cars will get their best mileage at
higher speeds.
Bigger,
heavier, less aerodynamic vehicles will get their best mileage at lower speeds.
This
is actually a pretty complicated question.
What
you are asking is what constant speed will give the best mileage. We won't talk
about stops and starts.
We'll
assume you are going on a very long highway trip and want to know what speed
will give you the best mileage. We'll start by discussing how much power it
takes to push the car down the road.
The
power to push a car down the road varies with the speed the car is traveling.
The
power required follows an equation of the following form:
road load power = av + bv² + cv³
The
letter v represents the velocity of the car, and the letters a, b and c
represent three different constants:
The
a component comes mostly from the rolling resistance of the tires, and
friction in the car's components, like drag from the brake pads, or friction in
the wheel bearings.
The
b component also comes from friction in components, and from the rolling
resistance in the tires. But it also comes from the power used by the various
pumps in the car.
The
c component comes mostly from things that affect aerodynamic drag like
the frontal area, drag coefficient and density of the air.
These
constants will be different for every car.
But
the bottom line is, if you double your speed, this equation says that you will
increase the power required by much more than double.
A
hypothetical medium sized SUV that requires 20 horsepower at 50 mph might
require 100 horsepower at 100 mph.
You
can also see from the equation that if the velocity v is 0, the power required
is also 0.
If
the velocity is very small then the power required is also very small.
So,
you might be thinking that you would get the best mileage at a really slow
speed like 1 mph.
But
there is something going on in the engine that eliminates this theory.
If
your car is going 0 mph your engine is still running.
Just
to keep the cylinders moving and the various fans, pumps and generators running
consumes a certain amount of fuel.
And
depending on how many accessories (such as headlights and air conditioning) you
have running, your car will use even more fuel.
So
even when the car is sitting still it uses quite a lot of fuel.
Cars
get the very worst mileage at 0 mph; they use gasoline but don't cover any
miles.
When
you put the car in drive and start moving at say 1 mph, the car uses only a
tiny bit more fuel, because the road load is very small at 1 mph.
At
this speed the car uses about the same amount of fuel, but it went 1 mile in an
hour. This represents a dramatic increase in mileage.
Now
if the car goes 2 mph, again it uses only a tiny bit more fuel, but goes twice
as far. The mileage almost doubled!
Efficiency
of an Engine
In
effect the efficiency of the engine is improving. It uses a fixed amount of
fuel to power itself and the accessories, and a variable amount of fuel
depending on the power required to keep the car going at a given speed.
So,
in terms of fuel used per mile, the faster the car goes, the better use we make
of that fixed amount of fuel required.
This
trend continues to a point. Eventually, that road load curve catches up with
us.
Once
the speed gets up into the 40 mph range each 1 mph increase in speed represents
a significant increase in power required.
Eventually,
the power required increases more than the efficiency of the engine improves.
At this point the mileage starts dropping.
Let's
plug some speeds into our equation and see how a 1 mph increase from 2 to 3 mph
compares with a 1 mph increase from 50 to 51 mph.
To
make things easy we'll assume a, b and c are all equal to 1.
|
Speed |
Equation |
Result |
|
3 mph |
3+3²+3³ |
39 |
|
2 mph |
2+2²+2³ |
14 |
|
Power Increase |
25 |
|
|
51 mph |
51+51²+51³ |
135,303 |
|
50 mph |
50+50²+50³ |
127,550 |
|
Power Increase |
7,753 |
|
You
can see that the increase in power required to go from 50 to 51 mph is much
greater than to go from 2 to 3 mph.
So,
for most cars, the "sweet spot" on the speedometer is in the range of
40-60 mph.
Cars
with a higher road load will reach the sweet spot at a lower speed. Some of the
main factors that determine the road load of the car are:
Coefficient
of drag.
This is an indicator of how aerodynamic a car is due only to its shape.
The
most aerodynamic cars today have a drag coefficient that is about half that of
some pickups and SUVs.
Frontal
area.
This depends mostly on the size of the car. Big SUVs have more than double the
frontal area of some small cars.
Weight. This affects the
amount of drag the tires put on the car. Big SUVs can weigh two to three times
what the smallest cars weigh.
In
general, smaller, lighter, more aerodynamic cars will get their best mileage at
higher speeds.
Bigger,
heavier, less aerodynamic vehicles will get their best mileage at lower speeds.
If you drive your car in the "sweet spot" you will get the best possible mileage for that car. If you go faster or slower, the mileage will get worse, but the closer you drive to the sweet spot the better mileage you will get.
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