I'm having trouble convincing my dad + sister that..

[BoberFett]

The best mileage will come from the lowest RPM in the highest gear. It's all about gear ratio.

 

[Siddhartha]

My 1987 Honda CRX with a 72 hp 1.6 carburetored engine over the same road at the same time of year got 43 mpg at a steady 55 mph and 41 mpg at a steady 70 mph.

YMMV

 

[SilthDraeth]

I know this thread hasn't been replied to in a while, and damned if Zenmervolt didn't school all of us.

I would like to ammend my post about breaking the shaking point barrier. I believe that is related to drag, or any other external force, but that asside.

In easy to understand terms for everyone else out there, think about this.

RPM=rotations per minute.
Engine runs at 2k rpm in idle. So ever minute, its crank shaft has rotated 2000 times.

If you drive a 4cyl car, then every cylinder has fired 500 times.

If your 4 cyl has a 2.0 litre engine, then everytime it fires it uses 2.0 litres of uncompressed vaporized gasoline/oxygen mixture.

Leaving out compression ratios, and dealing with liquid measurements, for simplistic sakes, 2x2000=4000 litres. Increase RPM, you increase the amount of LITRES of gas used.

So, highest gear, to maintain your current driving speed, at the lowest rpm, would sound like the best way. Problem is you can't just shift into 5th gear and start driving, but if you run through 1, 2, 3, you can probably then drop it into 5th and cruise around at 35 mph. Heck I used to cruise at 20, in 4th gear. My foot barely touched the accelerator.

 

[DrPizza]

Question: has there been actual research on this since the 1970's?

 

[cavemanmoron]

Originally posted by: jagec
It totally depends on the engine, the gearing, and the aerodynamics of the car. All cars have their own most efficient speeds, but they tend to be designed around ~60MPH, I'd say.

Agreed;
48-68 mph is normally, best for fuel economy.

I have noticed with more than one vehicle,
that 65-70 mph cruise, uses less fuel than 70-75+.

32 years worth of doing my own driving.

 

[blakeatwork]

my Firefly gets it's best mileage in the 80-90KM/h range...

 

[TomKazansky]

It's not the speed, but the gearing of the car that saves you gas. Obviously if you're driving on the freeway you are cruising at the highest gear while your engine is moving with the momentum of the car (assuming you are doing 10+ legal speed limit MAX).

 

[fuzzybabybunny]

Originally posted by: Zenmervolt
ZV

Proof?[/quote] http://www.roadandtrack.com/article.asp...ion_id=20&article_id=3424&print_page=y

Note that the author is a member of the Society of Automotive Engineers.

From the article (emphasis added):

"The most efficient way to reach cruising speed is wide-open-throttle (WOT) short-shifting. That is, not only do revs cost money, but so does prolonged motoring in lower gears, when throttling and pumping losses are their greatest.

WOT/short-shifting can save as much as 20 percent in city driving, worst to best case. In actual practice, rarely does traffic allow full WOT, but it's certainly fun ? and efficient as well ? to accelerate briskly through the lower gears to whatever the ambient speed happens to be.

Once there, the appropriate choice of gear is the one that offers modest rpm with relatively large (and constant!) throttle."

ZV[/quote]

I think that you have to explain a few points.

Note that all my personal experiences are from driving a Toyota Echo stick.

1. AC vs. Windows Down
Author argues for windows down, but I've heard that modern AC systems are so efficient now that turning on the AC will decrease your fuel mileage by very little, perhaps well under 5%. With personal experience, I've found that air conditioning doesn't effect my mileage hardly at all, and I have it on all the time. My average mileage is 42.5 MPG. The AC argument is now comfort vs. a very small percent drop in fuel efficiency.

2. WOT is best.
Is the author trying to say that it's always best to do your "0-60" acceleration to your cruising speed?

My 1st gear goes from 0-30mph. 2nd gear goes up to 60mph. Is the authoer saying that when entering a highway, the best thing for me to do is redline 1st and 2nd, hit 3rd for a bit to get to 70, then shift directly to 5th for cruising without even touching 4th?

And what does he mean that "the appropriate choice of gear is the one that offers modest rpm with relatively large and constant throttle?

What is "modest?" My car redlines at 6500 RPM. Is he saying it's best to stay around 3250 RPM?

I always thought 2000RPM was my most efficient RPM for flat road usage. RPMs higher than 2000RPM become most efficient only during an incline because then the engine spins easier up hills. Likewise, RPMs below 2000RPM are NOT good for cruising because the engine is having a tougher time turning, and thus burning more gas, than if it was at its relatively easier-spinning, but faster, 2000RPM.

 

[EagleKeeper]

The lower RPMs per MPH the better fuel economy.

Shifting up as soon as the vehicle has the momentum to handle the next gear will help keep the RPMs lower as you increase speed.

On a 5 speed, I can usually go from 2nd to 4th or 3rd to 5th without any real effort at 2000 RPM. If I can hold 2000 RPM at 60 - 70 MPH, that seems to be the best economy for my vehicle.

 

[soccerballtux]

The least fuel is used when the engine is running at the lowest RPMs. That would be at the beginning of 5th gear.

 

[Tom]

That article's point about WOT sounds wrong to me. It would be right if a car was designed with a specific rate of acceleration and maximum speed, but the vast majority of cars are capable of operating at much higher speeds than their most efficient cruising speed, so using the excess acceleration would be a waste of fuel.

I'm sure that accelerating WOT to 65 mph in my Hemi Dodge would be fun, but would use an enormous amount of fuel compared to some optimal efficiency rate of acceleration, which might be exaclty the opposite of WOT, ie the absolute minimum amount of throttle required to accelerate at all.

 

[Quasmo]

I drive a Nissan Pathfinder, and I get about 19mpg while driving 77mph. That levels off at about 3000rpms. At 2100rpms I'm going about 55mph. As I see it there is very little fuel economy lost from the jump from 55 to 77 both are about 70% of eachother. So I consider the very minimal gas I'm saving against my time, and the fact that I run the AC, so running the AC for 3.5 hours instead of 5 hours is alot better.

 

[Specop 007]

I have a Jetta TDI.
At 100 I get around 33
At 80 I get around 38
Around town I've gotten over 40.

 

[jupiter57]

It all depends on the vehicle:
My truck: the faster you go, the more the mileage drops (~9 MPG).

Wife's DeVille: Gets better mileage at 70-75 than at 55! (21 MPG @ 55,-27 MPG @ 75)

 

[fbrdphreak]

Originally posted by: virtualgames0

Originally posted by: AMDUALY

Originally posted by: virtualgames0

Originally posted by: Zenmervolt

Originally posted by: virtualgames0

Originally posted by: Zenmervolt

Originally posted by: virtualgames0

Originally posted by: Cattlegod
techically wouldn't the most efficient speed be as follows:

highest gear
lowest hp rpm on the car (probably just above idle)

most cars that ends up being around 40 mph in a 5 speed or so.

That's a common misconception.
Engines are most efficient at 2500-3500RPM
If you bring it below that, you are wasting gas.

Wrong. RPM is the largest player in fuel consumption. Throttle opening is vastly more important. It's more efficient to run 50% throttle at 1,800 RPM than it is to run 25% throttle at 3,000 RPM.

You can claim it to be a misconception all you like, but testing proves your view to be incorrect.

ZV

Proof?

http://www.roadandtrack.com/article.asp...ion_id=20&article_id=3424&print_page=y

Note that the author is a member of the Society of Automotive Engineers.

From the article (emphasis added):

"The most efficient way to reach cruising speed is wide-open-throttle (WOT) short-shifting. That is, not only do revs cost money, but so does prolonged motoring in lower gears, when throttling and pumping losses are their greatest.

WOT/short-shifting can save as much as 20 percent in city driving, worst to best case. In actual practice, rarely does traffic allow full WOT, but it's certainly fun ? and efficient as well ? to accelerate briskly through the lower gears to whatever the ambient speed happens to be.

Once there, the appropriate choice of gear is the one that offers modest rpm with relatively large (and constant!) throttle."

ZV

Ummm that article says nothing about keeping the RPMs low.
All it says is to accelerate to speed limit quickly so you don't use the low gears as much as it causes engine wear. If anything, that means revving up higher.

It says to get through your low gears quicker so that you don't spend as much time revving in them (RPMs). You need to get through your lower gears so that you have speed to MOVE in the higher gears. Lower gears rev higher so get through them quickly so y ou can cruise in higher gears which don't rev as high. Rev = RPM = Waste gas = You fail Reading comprehension

That's basically what I just said..
you fail reading comprehension.

No, it really isn't. Just give it up

 

[cKGunslinger]

Who cares? Gas is cheap!