atomicskiier01

I was driving down I-65 to go to the Purdue vs. Indiana football game (The Boilers kicked hiney, BTW) and I was playing around with the "Average Fuel Economy" on my 06 GT Manual. I was always under the impression that you get the best fuel economy on the highway in 5th gear, and the faster you go the better (Without getting insane). I reset the system and started trying out different speeds. I found that If I went about 60-65, I could get it over 25 mpg, but when I sped up to 80 and cruised there, it would shoot down to about 21 mpg real quick and I could hear the engine working harder as the rpms went up. Is that just the nature of the manual V8, or does something seem fishy here?

Clem

Wind resistance at 80 is much more than at 60-65. Sounds about right to me.

dustindu4

I get 29 mpg at 55 mph

Rogue6

On my manual V6, I noticed that if I keep it between 65-70, I get about 30-32 MPG. When I hit 80 though, I drop to 26-27...darn the laws of drag!

red454

On a two hour trip I averaged 60 mph and got 27.5 mpg. Drove conservatively - no throttle mashing.

GhostTX

Even though I haven't been tracking my numbers precisely, I've found the "sweet spot" for mileage is 55-65mph.

Darn if its so freakin' hard to drive that "slow" though!!!

atomicskiier01

I agree....its hard to determine whether or not my time is worth the amount of money that I am saving by driving at that speed....

Thanks for the replies, the wind resistance makes sense to me.

Eleanor Dreaming

As far time savings 60 is a mile a minute, if my math is correct which if it is wrong then the calculater in my PC is wrong, 75 is 1.25 miles per minute. If mileage goes down at 75 enough that to save a gallon or 2 over a trip or tank then my time is worth that especially at some of the recent prices. I just leave earlier.

Last time I checked more horsepower more speed almost always means more fuel usage. You usually don't see race drivers that are asked to save fuel go faster. Oh, and yes in some situations it makes the car handle better and they get though the turns better

GhostTX

Yes...for traveling long distances it would well be worth it to stretch every mile...and at the same time, how much time would you save over the course of the trip? Over a four hour trip...going 75mph saves you an hour on the road at 60mph. All up to you.

City driving though...pretty much all MPG goes out the window. 60...75mph won't matter too much in 20 minute sprints.

ski

I once read that 45 mph is the theoretical optimum speed in order to get the max mpg in top gear, assuming that the vehicle is traveling on a level road with no hills.

Also, air drag does not vary linearly to changes in speed, but instead varies in proportion to the square of speed changes(assuming all other variables remain constant).
This means that the drag at 80 mph will be 4 times as great as the drag at 40 mph.

atomicskiier01

An extra hour? I'd rather spend the cash....Although an hour in my 'stang is much nicer that an hour in my '96 Cavalier

Firedude

Your numbers are right where mine are. 65 is the sweet spot for mileage. 85 feels sweet though. 125 feels even better. 140 feels..... well, you get the picture I'm sure.

jwenger

Car companies, in order to attain their highest highway miles figure, drive on smooth straight roads at 48mph steady and in high gear. It isn't what you'd do but to answeryour question this is the optimum way to achieve the very best milage.

AFBLUE

I remember reading something similiar that 45-50mph gave the best mpg.

Here's a cool webpage discussing this issue http://auto.howstuffworks.com/question477.htm

"the "sweet spot" on the speedometer is in the range of 40-60 mph" It will very depending on weight, frontal area, and drag coefficient of the vehicle.

TehSLeeper

I wouldn't neccesarily equate it to drag, I think it has more to do with the how fast the engine is turning. The faster the engine is turning, the more fuel it needs.

ski

Thanks for the link, AFBLUE. Some real good stuff there.



Actually, drag is the primary factor in a vehicle's gas mileage assuming it's traveling at a constant speed on a level road.
And you're right about the engine's rpm's also affecting gas mileage. However, is it logical to drive at 40-50 mph on a level road with the engine racing in 1st gear when 5th or 6th gear gets the job done?

Trojan2002

Not that I usualy drive this slow, but I reset my mpg meter and went 45 mph in 5th...meter said 33 mpg! But if I was worried about gas mileage, I would have bought the V6. I am more than satisfied with my 21.5 city/hwy mileage.

Route 66

I'm so glad we don't have national speed limit laws anymore. But that's why the environmentalist wacko's want a speed limit of 55 mph on interstates. Saves gas.

gmichael

I believe you'll find about 48 mph to be optimal. This is where the manufacturer sets the last shift point on an automatic transmisssion to get the highest fuel economy numbers.

Whoops- as said earlier

atomicskiier01

Thanks for all of the good info!!!!