[Leanan]

CNN is running a story on this today. Some people have been trying to say that today's cars are just as fuel-efficient at 75mpg as at 55mph, because they are tweaked to perform at higher speeds. They claim the studies that say 55 is best are old, dating from the '70s.

Not true. They've done new studies, and fuel efficiency still peaks at 50-55mph for cars. The problem isn't the engine, it's the aerodynamics. Faster than that, and air resistance causes a lot of drag.

It's even worse for boxy vehicles like SUVs. An SUV hits maximum fuel efficiency at 45mph.

[UIUCstudent01]

It's even worse for boxy vehicles like SUVs. An SUV hits maximum fuel efficiency at 45mph.

That's no good: I see SUV's hitting about or above 60 mph all the time at my hometown between suburbia and exurbia. They have even lower fuel efficiency because of that

[pilferage]

I googled and caught this off a hybrid discussion forums:

Cruising at 55mph in normal summer weather netted me an average of 53.5 mpg. Cruising at 70mph nets an average of 46.5 mpg. That 's a huge difference, caused by higher engine rpm (2200@55 versus 2900@70 I think) and aerodynamics.

To get the same mileage at 55 and 75, the transmission would need to be geared st the increase in torque (efficiency) from 55 to 75 would have to compensate for the increase in energy expended due to drag. But, this might occasionally be the case because I've noticed people that own some of the newer larger disp suv's and sedans claim to get slightly better mileage (maybe 2 mpg better) if they travel at 75 vs. 55, but for most cars, I'd say that 40-60mph is the optimum speed for fuel efficiency. My 92 v6 Camry for example goes from ~37mpg to ~30mpg if I go from 55 to 75. It seems like a big YMMV, but like I stated, as a whole, the optimum is probably ~55mph.

[Frank]

A vehicle would have to be extremely poorly designed or tuned if mileage was better at 75 than at 55! It takes more force ergo power to move anything at the higher speed and more fuel has to be burned to do it. If an engine had an extremely radical camshaft I could see this happening but I've never heard of such a thing. The SUV drivers who say they get better mileage at higher speeds are full of crap.

[Tyler_JC]

My car has super fuel efficiency. I leave it in the driveway and the movement of the tectonic plates moves it along without me having to burn any gasoline. Talk about a free lunch!

I’m inclined to believe that ~55mph is the best speed for fuel efficiency. Especially because fewer accidents occur at that speed and accidents are very energy spendy. After all, ambulances, police cruisers, insurance agents, and clean up crews use lots of gas. Plus, you have to replace the car (assuming you survive the horrible car crash that turned your sedan into a crushed soda can).

[Bytesmiths]

Air friction is a second or third order function: energy required goes up between the square and the cube of velocity.
In layman's terms, this means that it always takes a lot more energy to go faster. Period.

The <b>only</b> way a vehicle can get better efficiency at higher velocity is if the engine's power band or transmission is so poorly designed that relatively small changes in RPM result in huge differences in efficiency. With modern five and six speed transmissions, that seems very unlikely.
The people who tell you they have better fuel economy at 75 are deluding themselves. They just like to drive fast, and will lie to themselves to justify it. This is "faith based" fuel economy!

With Veggie Van Gogh, I get about 13 mpg at 65, but nearly 17 mpg at 55, and nearly 20 mpg at 45. (Not bad for a 10,000 pound vehicle!) This is with eight forward speeds, so I can hit the engine's peak efficiency point (about 1700 rpm) at each of those speeds.
Veggie Van Gogh is a big sail, and so energy is probably is closer to velocity cubed. A streamlined vehicle would be closer to velocity squared. But even if energy were linear with velocity, it would still go up as you drive faster!

Many manufacturers have made vehicles that are sleek in the front. What has been relatively neglected is tail turbulence. Both styling and practical concerns (like rear entry) conspire against truly streamlined vehicles. Sleek in front and boxy in back still equals high drag! Ask any sailboat designer!
BTW: Tyler_JC has the right idea. Based on commercial diesel fuel that I use, Veggie Van Gogh gets well over 200 miles to the gallon of fossil fuel!

[ArimoDave]

Something occured to me while reading another thread -- http://www.peakoil.com/fortopic8326.html

mrniceguy posted:

At the same time there should be a policy of educating drivers in ways of driving more efficiently e.g. many people just don't realise that accelerating hard away from traffic lights uses more fuel than gently pulling away.

Is this really true?
First, consider that braking without regeneration, disipates energy without any recovery.
Also, think about all the cars behind the leader at the light. If the lead cars do not accelerate
fast enough, there will be a number of cars that will have to stop again (using up energy),
and leave their cars idling while waiting for the next signal change (using still more energy
without gaining distance.) The cars that get up to speed, usually -- in my experience -- can
get into the timed signal sequence better and keep moving, thus they do not have to stop.
Second, does it really take that much more energy to accelerate relatively quickly for a short lengh of time than it does to acclerate slowly for a long length of time?

As far as the theroetical energy to acclerate goes (say from 0 - 35 mph) , there is no net energy savings to do it rapidly over slowly. Any difference will come from the energy efficiency at the required rpm and power of the engine. A small, low-powerengine will likely be much more efficient acclererating rapidly than a large high-power engine acclerating slowly.

Does anyone know exactly how much less efficient most IC engines are when accelerating.
I have a feeling that there is an optimum accleration rate which is between a slow acceleration, and stomping on the pedal to the floor. (I happen to be in the last category, but I have a small engine -- I generally can't get the tires to break traction on dry pavement.)
I also find it frustrating when I am somewhat near the rear of the pack at a stoplight and
cannot make it through the green light because the cars at the head do not accelerate quickly
enough.

[pilferage]

Eh, it's kinda complex. Pick a specific car and I could go into detail about what the most efficient driving habit would be.

mrniceguy posted: At the same time there should be a policy of educating drivers in ways of driving more efficiently e.g. many people just don't realise that accelerating hard away from traffic lights uses more fuel than gently pulling away.

Is this really true? [/quote]
Generally, yes.

First, consider that braking without regeneration, disipates energy without any recovery.
Also, think about all the cars behind the leader at the light. If the lead cars do not accelerate fast enough, there will be a number of cars that will have to stop again (using up energy),

It sounds like you have some craptacular stoplights. The ones out here usually stay on long enough to allow everyone waiting through.

and leave their cars idling while waiting for the next signal change (using still more energy without gaining distance.) The cars that get up to speed, usually -- in my experience -- can get into the timed signal sequence better and keep moving, thus they do not have to stop.

What you have here is something of a trade off. Lets say I travel at 55mph, and I'm able to catch green lights 60% of the time vs. someone who travels at 35mph and only catches green lights 40% of the time. Since we mostly brake instead of coasting to a stop, the car @ 55mph is losing more energy when it comes to a stop compared to the car @ 35mph. However the car @ 35mph stops 33% more compared to the car @ 55mph.
The question is, which car loses more energy? The one losing more energy but getting more greens, or the one getting more reds, but losing less energy?
Generally, there's some optimum speed to travel, which depends on traffic/light patterns and the efficiency of your car at whatever speed minimizes the amount of energy you lose from breaking.
To put it in a nut shell, coast as much as you can. Avoid braking because it robs you of kinetic energy and decreases your gas mileage.

Second, does it really take that much more energy to accelerate relatively quickly for a short lengh of time than it does to acclerate slowly for a long length of time?

It depends on a few things, your car's engine, weight, and coefficient of drag.

A small, low-power engine will likely be much more efficient acclererating rapidly than a large high-power engine acclerating slowly.

It may or may not be, it really depends on the engine and what you mean by rapidly and slowly.

Does anyone know exactly how much less efficient most IC engines are when accelerating.

IC?

I have a feeling that there is an optimum accleration rate which is between a slow acceleration, and stomping on the pedal to the floor.

On modern fuel injected engines do not floor it if you want good gas mileage, the engine will enter open loop mode and your mileage will suffer.

I also find it frustrating when I am somewhat near the rear of the pack at a stoplight and cannot make it through the green light because the cars at the head do not accelerate quickly
enough.

If you don't expect to make it thorugh the light then simply use very little gas to approach it while you wait for the next green.

[ArimoDave]

I found a few of my answers here.
I downloaded it in PDF, so I couldn't make any quotes from the paper.
The upshot, it seems, is that slow acceleration for an individual vehicle is more efficient.
However, this style of driving produces slightly more NOx than the "Sport" style. Also, the difference from the "Sport" driving style (i.e. putting the pedal to the metal) is only slightly worse than the "normal" driving style as far as economy goes.

But, as pilferage points out, the more you can keep moving, the better economy you will get.
For your information, IC = internal combustion. I had to figure this out a while back too, when one of the moderators (I think it was Jack) used it in a reply to me on another thread.
By the way, any and all comments about driving habits and their efficiency are welcome on this thread.
ArimoDave

[pilferage]

The upshot, it seems, is that slow acceleration for an individual vehicle is more efficient. However, this style of driving produces slightly more NOx than the "Sport" style. Also, the difference from the "Sport" driving style (i.e. putting the pedal to the metal) is only slightly worse than the "normal" driving style as far as economy goes.

Strictly speaking, slow acceleration is not more efficient for most older GIC engines, since the optimum driving in terms of efficiency involves accelerating with the throttle as open as one can without sending your ecu into open loop up to (or a bit after) your peak torque (where your engine is most efficient).
However, like I said, if I accelerate to 55mph and stop versus accelerating to 35mph, then I just wasted all that energy getting to 55mph, even if my acceleration was more efficient.

Newer engines equiped with variable valves and cams (this study probably doesn't cover them), can produce near peak torque throughout most of the rev range, so accelerating slowly is definitely better for them.
Ideally, the best driving strategy involves finding out two things. At what speed do you get the best mileage on the street and highway?

From there you drive based on situation. Rolling through stop signs increases gas mileage, as does never coming to a complete stop. When coming to a red light, try to brake st (such that) you maximize the speed at which you eventually move through the light (thus minimizing lost energy). Never ever accelerate towards a red, always coast. Pick a speed suitable for the distance between (possible) stops.
i.e. if there are two lights 1/4mile apart, I will accelerate slowly between them and usually pass the second light going ~25-30mph. But if the next light is 4 miles away I will accelerate using almost full throttle until ~35-45mph and cruise untils I see the light (then coast and/or brake).
The same applies to the freeway, except now you'd use your optimum freeway speed (for me, 55mph)

Using these techniques, I get ~30-40% better mileage than I did before I started using them.
Also, slightly overinflating your tires, using synthetic motor oil, gapping your spark plugs, removing excess weight, minimizing the use of accesories (A/C, headlights, radio), (possibly) using an appropriate amount of premixed acetone/gas, and above all, making sure your car is tuned up can help with gas mileage.

[Andy]

Drive a manual diesel car (no throttle) and operate it like a semi truck driver operates his truck when loaded. Coast in gear to stops etc. No fuel is consumed when coasting in gear and brake wear is minimized.

[k_semler]

Well, this is far from the most scientific analysis, but if I travel at speeds I am comfortable with on the highway, my MPG is 17.2. If I drive at 55 MPH, (5 MPH below the speed limit), my MPG increases to 18.43. I drive a 1981 Oldsmobile Delta 88 Coupe with a 350 and a 4bbl carb. 141,750 miles on the car now.

[Carmiac]

I have a '85 Honda Magna 700 in very good repair, no modifications to the carbs, engine or exhaust systems. Carbs were rebuilt about six months ago and balanced. For those of you who don't know, this is a middle weight cruiser style bike with the engine from Honda's sport line of the time, a 90deg V4 with a seperate carb for each cylinder.

Anyway, my mileage varies anywhere from 30 to 50mpg in town depending on how I am driving. If I accelerate and brake at conservative car-like speeds I get about 50. If I accelerate and brake at what the bike is really capable of doing (0-60 in 3sec, every stop light) I get 30.

Driving habits make a huge difference.

[Rosco]

We have two stock Saturn SL/2s in our family.
One is a 1996 and the other a 1999. Both of
these get ~35mpg in town and ~44mpg on the highway.

After 1999, the hiway gas milage for
Saturn seems to have taken a downward jump to the 35mpg range.

Why? Was this the time when GM took over day-to-day management at Saturn?

[turmoil]

in most futuristic movies (like star wars) most cars are moving at slow, but constant, speeds.

Maintaining 20-25 mph uses a lot less energy and gets you where you are going in the same amount of time than accelerating to 40, braking to 0-10, then accelerating to 30-40 again. It's D=RT "stupid."

Moral of the story: drive slower in fuel efficient cars/motorcycles/mopeds.

[redfire]

I've found for highway cruising, the best fuel economy is obtained using the cruise control. The speed is constant, but I'm surprised that the rpm's of the car I drive don't change even going up moderate inclines. I was first surprised with this, thinking the rpm's would have to increase. I think what's going on is that the slight load changes on the engine are being compensated by the timming changes controlled by the car's computer.

I've noticed at car shows that the vehicles with higher torque engines combined with taller gearing sometimes get better mileage than smaller displacement engines.

I think for stop and go city driving, if you are generally moderate in the way you drive and look ahead to anticipate what's going on, you'll get the best mileage.

[thegrq]

Fuel efficiency is of course better at highter speeds (ex: highway speeds), but only to a certain point. Wind resistance roughly doubles between 55 MPH and 70 MPH.

[dooberheim]

It's like riding a bicycle.
If you accelerate slowly, cruise at a moderate speed, and minimize stopping and starting you will be less tired/sweaty when you get where you are going than if you ride hard. Same for the engine in your car.
I remember from the '70s that we were told to drive like we had an egg between our foot and the accelerator. Seems to work for me, as well as cutting down on vehicle wear and tear.

[bobeau]

I've found for highway cruising, the best fuel economy is obtained using the cruise control. The speed is constant, but I'm surprised that the rpm's of the car I drive don't change even going up moderate inclines. I was first surprised with this, thinking the rpm's would have to increase. I think what's going on is that the slight load changes on the engine are being compensated by the timming changes controlled by the car's computer.

Um, timing is continuously variable, regardless of cruise control of whatnot. The computer calculates it based off a map. When someone talks about "raising timing" they're talking about a base offset, not actually what the engine sees.
When you go up an incline the CC gives more throttle. There is absolutely no reason why RPMs should change if speed is constant unless you have an auto that downshifts.

I've noticed at car shows that the vehicles with higher torque engines combined with taller gearing sometimes get better mileage than smaller displacement engines.

[/quote]
Engines roughly use 1/3 of the power produced to drive themselves. So getting more lower rpm power does help considerably. Reason #1 why diesels are so efficient. Reason #2 is the high compression ratio allowing for higher thermal efficiency - since heat waste goes to roughly another 1/3 of power produced.

I think for stop and go city driving, if you are generally moderate in the way you drive and look ahead to anticipate what's going on, you'll get the best mileage.

Sorta. The rate of acceleration in itself really makes a marginal difference. It's the braking that's wasteful.

[rs]

When we had the fuel protests here in the UK in 2000 I noticed after a week a dramatic change in peoples driving habits (obviously due to difficulty obtaining fuel!).

Everyone suddenly was driving around within the speed limits. Few were driving over 50mph on dual carriageways etc..

No boy racers about! Everyone more courteous. In fact, I can't remember a time when driving was so pleasurable, ironic huh!