The same principle can be readily illustrated on a mountainbike. Pump the tires up to the maximum on the sidewall and ride around the block. Then lower them to, say, half that and again ride around the block. You’ll be amazed at how much more energy it takes to pedal the bike when the tires are at the lower setting. Imagine that applied to your car weighing a few thousand pounds or more and you can easily imagine how much more energy it takes to keep it rolling when the tire pressure is low.
If you only check your fuel mileage once in a while you can be fooled into thinking you are getting much better mileage than you are. If you are travelling in the same direction as the prevailing wind, it can really boost your mileage.
A good point re winds. Driving at 60 MPH with a 20 MPH headwind will lower your mileage a lot. The wind resistance, which is most of the drag, is equivalent to 80 MPH even though you are covering distance at only 60 MPH, and drag goes up as to the square of the speed.
That means drag is about 1.7 times higher, and gas mileage would drop by about that factor, if I did the math correctly.
Yes, aerodynamic drag force is proportional to velocity squared, but power (and therefore fuel consumption) is proportional to velocity cubed.
Didn’t know that, thanks… That makes the factor 2.4
(80/60)³
??? I view fuel consumption as gallons per hour.
Regarding overcoming aerodynamic drag:
power = force X velocity
force = (constant) X velocity squared
So, power = (constant) X velocity squared X velocity = (constant) X velocity cubed
Please feel free to correct as needed.
NOTE: The above applies only to aerodynamic drag.
The following from mpgforspeed.com seems reasonable:
According to studies backed by the department of energy, the average car will be at its advertised MPG at 55 mph. But as the speed increases:
- 3% less efficient at 60 mph - 8% less efficient at 65 mph - 17% less efficient at 70 mph - 23% less efficient at 75 mph - 28% less efficient at 80 mph
How you view it is unambiguous to you, but ambiguous to others.
BillRussel took it to mean fuel consumption per unit distance (i.e. MPG) and got it wrong.