[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]

$this->bbcode_second_pass_quote('', 'I')t's even worse for boxy vehicles like SUVs. An SUV hits maximum fuel efficiency at 45mph.

[pilferage]

I googled and caught this off a hybrid discussion forums:
$this->bbcode_second_pass_quote('', 'C')ruising 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.

[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:$this->bbcode_second_pass_quote('', ' ')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.

[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.
$this->bbcode_second_pass_quote('ArimoDave', 'm')rniceguy 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.

[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]

$this->bbcode_second_pass_quote('ArimoDave', 'T')he 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.

[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]

$this->bbcode_second_pass_quote('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.

[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!