Odd question on milage and wind

fuel-economy

#1

I have to drive 100 miles round trip to work. I know slower speeds will get better milage. I have done this 5-6 times a week for 8 months, so the following numbers are pretty stable. My current mileage is (up and down long hills): 36mpg at 60 mph and 29mpg at 75mph



Now the odd question: during high head winds 65-75 mph (not unusual), I get 22mpg whether I drive at 60 or 75 mph. Why is the milage the similar at both speeds?

It is obviously safer to drive at slower speed due to the extreme buffetting wind.


#2

One way to look at it is “aeronautically”: you have ground speed and air speed. Above ~ 60mph, overcoming air resistance becomes an increasingly significant external factor influencing mileage. As far as how hard the engine has to work to overcome air resistance, driving 60 mph into a 30 mph headwind would mean an airspeed of 90 mph – you might as well be driving 90 mph with no headwind, and mileage would be reduced accordingly. Conversely, driving 90 mph with a 30 mph tailwind would get mileage numbers as if you were driving only 60 mph.

A second factor is rpm. If headwinds are so severe you have to downshift to maintain even a moderate speed, that would significantly affect your mileage (e.g. if you have to downshift to 3rd gear just to maintain 60 mph).

The optimal speed for best mileage is the slowest you can go in the highest gear without lugging the engine. Different vehicles are geared differently.

For example, a 1994 Ford Ranger V6 w/ 5spd MT would achieve optimal mileage at 41mph (owner’s manual).

A 2001 Honda Civic HX VTEC-e w/ 5spd MT would achieve optimal mileage at 48 mph (again, owner’s manual).

With, say, the Honda, if you introduce a 30 mph headwind into the equation, you would not be able to maintain 48 mph without downshifting, so the engine would be both working harder and spinning faster just to maintain even that moderate speed.


#3

I would have to guess that there really is a difference when driving faster or slower into that head wind, but it is a smaller factor because of the wind. Aerodynamics is a complex issue. There are very many factors adding into the mix. With a complex shape of a car you are likely to find sweet spots that is certain wind speeds that may have less drag at a slightly higher speed than slightly lower. Specific RPMs etc all play in the mix as well, so predicting based on formula is not always very accurate, which is why the manufacturers use wind tunnel test.

That said, I would guess there is something else coming into ply with that 22 mpg number for both 60 and 75. Maybe a difference in the actural wind speed or a measurement error etc. In other words, I expect you are still saving at 60 vs 75.


#4

One way to look at it is “aeronautically”: you have ground speed and air speed. Above ~ 60mph, overcoming air resistance becomes an increasingly significant external factor influencing mileage. As far as how hard the engine has to work to overcome air resistance, driving 60 mph into a 30 mph headwind would mean an airspeed of 90 mph – you might as well be driving 90 mph with no headwind, and mileage would be reduced accordingly. Conversely, driving 90 mph with a 30 mph tailwind would get mileage numbers as if you were driving only 60 mph.

This is true for aircraft but does not exactly fit the situation with ground vehicles which always push against a static ground to propel themselves.
To really see how these two situations differ, consider a ultralight with a 50 mph top speed flying against a 50 mph headwind, it needs to run at full power just to stay stationary with a 50 mph airspeed.
On the other hand, a car can stay motionless in the same 50 mph headwind with the engine off and the brakes locked.
It takes a lot more power to go 90 mph in calm air than it does to go 60 mph against a 30 mph headwind. The force of overcome the air resistance is the same but the engine only has to go 60 mph against that force.


#5

Do you really have 60 to 75 mph wind where you live? If you can talk to someone without screaming in his ear, it wasn’t blowing 60 mph.
Let’s take a more realistic 40 mph wind, which most people will overestimate to be a 60 mph wind. Driving 60 mph against it gives you a 100 mph relative wind and 75 mph will give you a 115 mph relative wind, a 115% increase in wind means a 1.15 squared or 132% increase in resistance.
Going 75 instead of 60 in a calm is a 125% increase in relative windspeed and results in a 156% increase in resistance.


#6

These events didn’t really have to happen as described, and the data can be completely bogus. Treat this as a thought problem. The situation and responses are still quite interesting.


#7

the same change in efficiency is seen when fishing. if you cast down wind, the lure goes alot further than if you cast upwind.

so the same logic applies here.

a car driving into say a 10 mph head wind will have X amount of ‘friction’ due to wind.

the same car, same road same inclination, with a 60 mph head wind will have an increase in drag from increased friction X plus ?%).

this increase in drag is the same as driving up hill all day, the engine has to work harder to go up hill than it does going down hill. thus the mileage difference.

by crude figures, i believe the 75 mph head winds would more than double the resistance needed to overcome the increase in friction. actually i believe it would exponentially increase the resistance.

i think BLEs numbers are pretty understandable. of course, since math is not my friend, i cant vouch for them, but they look good.


#8

65-75 mph winds??? That’s low hurricane speeds!!! Where are you driving with that much wind.


#9

Yes realy 60-75 mph winds. Not common, but during the winter months 6-8 times a month. It can be +40 degrees and -45 chill factor with the wind. March of '07, they closed the road around Wheatland, WY because of 100mph cross winds and icy roads. In Casper and Douglas WY, wind doesn’t start until 30mph. 25 years ago, the average for the year was 45mph. And yes you do have to scream if you are in it and sometimes walk backwards because of the dirt it blows (hurts)