Best cruising speed for highest MPG.

[babydriver]

Originally Posted by Xenith

I recall reading in a Thermodynamics textbook this past Spring that the optimum speed for fuel efficiency is 55mph (typically) and then fuel efficiency decreases 10% every 5mph higher you go.

Yep. Why do you think that the U.S. highway speed limit was 55 mph during the 70's and early 80's? Gas crisis.

[babydriver]

Originally Posted by phrosty

If you granny drive, you can easily get 35. I got 36-38 during my break-in period, and that was about 50% city / 50% 75-85mph freeway. Helps to have an automatic.

The automatic is geared higher in the rear end, so yes, it gets better mileage. Not as much fun, though.

[babydriver]

Originally Posted by Tcoat

Anybody that coasts down a hill in neutral is a tool anyway! You just turn your nice shiny sports car into a soapbox racer with no hope in hell of reacting fast enough if something goes wrong and you need to power out of it!

Wait a minute! My soapbox derby racer won three heats before some boneheaded adult tore out the top rear panel behind the driver when loading it. Lost the next heat; big surprise due to the added turbulence from the GAPING HOLE! I've always suspected that his kid was in the race and he didn't want him to lose! So don't knock the soapbox derby.

[Tcoat]

Originally Posted by babydriver

Wait a minute! My soapbox derby racer won three heats before some boneheaded adult tore out the top rear panel behind the driver when loading it. Lost the next heat; big surprise due to the added turbulence from the GAPING HOLE! I've always suspected that his kid was in the race and he didn't want him to lose! So don't knock the soapbox derby.

Not knocking the derby in the least!
Knocking people that turn their car into one by going down hill in neutral.

[phrosty]

Originally Posted by babydriver

The automatic is geared higher in the rear end, so yes, it gets better mileage. Not as much fun, though.

Well yea, we're talking mileage here not fun

I get ~24mpg now because of sticky tires and "fun".

[campy]

Originally Posted by Tcoat

I get 32 on a regular basis. Dropped all the way down to 31 with the snows on and cold weather.
Mind you that is at 98% highway driving with cruise set at 75!

Yep I also find that setting CC to 75 yields pretty good gas mileage, typically above 30mpg according to the instantaneous MPG readout. Mind you I have FI and drive on hilly NJ highways, so this is very good in my opinion.

[MokSpeed]

I just push the car and I don't even waste gas.

[Bobblehead]

I've found 70 was a nice happy medium between gas mileage and not getting left in the dust by everyone else on the highway.

[Tcoat]

Originally Posted by MokSpeed

I just push the car and I don't even waste gas.

At least get help!

[53Driver]

Crickets or no crickets?

[ashtray]

Originally Posted by Tcoat

Not knocking the derby in the least!
Knocking people that turn their car into one by going down hill in neutral.

Wait, can I use my car in a soap box derby? Sure it would be pretty slow off the line but as I gain speed I can probably outcorner the other cars.

[Teseo]

I spank the gas pedal because its sport car and i dont care fuel economy.
But if im in the highway i try to keep speed at 55mph in 6th gear, if i have chance i try drafting with suv

[FRiSson]

Originally Posted by ArgentoAtl

I found a relevant article http://www.popularmechanics.com/cars...-this-weekend/.

Two questions grew from my reading of the article.

1) Your example of taking 7 seconds instead of 14 seconds makes some sense, but I did not see how to get that exact result from the link. Perhaps, you were just trying to give a rule of thumb you invented? The graphics in the article were a little hard to understand, imo, and they do not tell whether the tests were made with an automatic or a manual transmission, e.g., , but they do seem to suggest the general idea of your comment is correct.

2) The link article says that leaving the car in gear and descending a hill is more fuel efficient than putting the car in neutral and indicated that newer cars have a special fuel shutoff that works only when in gear and the accelerator is not engaged. First of all, do the twins have that feature? Also, one of the comments says that this turned out not to be true for the commenter who apparently experimented on a hill using the car's mpg sensor. Any comments?

Brian in Atlanta

Its from the article. I did not invent the idea. I assume PM did some testing to confirm the half-maximum rule. I do suspect that not all fuel systems respond the same - so results will vary. In particular, the twins have a very complex fueling system that involves both port and direct injection being involved at times, and not in a simple linear speed order. As far as going with the MPG sensor, that is an imprecise device. I am not saying it is necessarily wrong, but that it isn't sufficient evidence to go on.

[billwot]

The power (and fuel) required to overcome drag (air resistaance) rises as the cube of the increase in speed.

"I put 55 mph and 65 mph in this table to show why some people think that the 55 mph national speed limit saves gasoline. It only requires about 7 hp to overcome drag at 55 mph, while it requires almost 12 hp to overcome drag at 65. Fuel consumption is approximately proportional to horsepower expended."

http://phors.locost7.info/phors06.htm

So a 18% increase in speed requires 70% more power (and fuel)