Hypermileing: is slower better?

peeplj · Post by **peeplj** » Sun Oct 12, 2008 7:17 pm

Slashdot has an interesting articleon the relationship between speed and fuel efficiency.

The overall consensus seems to be that driving slower attempting to reduce fuel consumption on most vehicles actually reduces fuel efficiency.

I can confirm this with my experiences on my own car. It's a 97 Neon, and I've had years to experiment and get to know the engine and acceleration curve.

I get about 36 mpg on the highway, whether or not I drive 65, 70, or 75.

Above 75, mileage starts to decrease a little.

Under 60, mileage starts to decrease a lot...holding 55 reduces mpg on my car from 36 to 28.

Also, I've found (as mentioned in the comments on the slashdot article) that coming quickly to speed is more fuel efficient than trying to accelerate slowly, and that holding a steady cruising speed is more important to fuel economy than trying for a specific speed.

Anyone want to share their thoughts or experiences?

--James

s1m0n · Post by **s1m0n** » Sun Oct 12, 2008 7:24 pm

One the highway, the geatest efficiencies are likely to be by going whatever speed everyone else is, and collectively 'drafting' in one big pod.

chas · Post by **chas** » Sun Oct 12, 2008 7:29 pm

My last two cars have been Outbacks, and I don't see a significant difference between 65 and 70 (should be something approaching 20% according to conventional wisdom). I definitely get better mileage (according to the trip computer) at 45-50 than at 65-70. I've never driven several hundred miles at 50, so can't do actual calculations.

Another interesting bit that flies in the face of conventional wisdom is that I suffer at most 1 MPG or about 3% (according to my actual measurements of mileage over long trips, many trips over several years) by having a roof box on the car. Of course, in addition to the drag of the roof box, I'm usually carrying 100 lb or more extra in the car when driving with it on.

djm · Post by **djm** » Sun Oct 12, 2008 7:56 pm

I find driving slow causes me to suffer far more than the mileage does.

djm

Jerry Freeman · Post by **Jerry Freeman** » Sun Oct 12, 2008 9:02 pm

As far as I could tell, they weren't saying slower or faster is inherently better.

It sounds like they're saying, it depends on the car. If the rpm's are kept in the range where the engine's most efficient, that's where the gas mileage will be the best, assuming it's a small, aerodynamic car. In the example they gave, I think they said for that car the range was around 1,500 to 2.500 rpm's.

That's my reading of what they're saying. Is it true?

Best wishes,
Jerry

Innocent Bystander · Post by **Innocent Bystander** » Mon Oct 13, 2008 2:16 am

Of course it depends on the car.

Back when I did my car maintenance course, the instructor was at pains to point out that for most cars (then) peak efficiency was deliberately set at the "12 o'clock" position on the speed indicator. It might be 50 mph or 70 mph for your car, but that was your best fuel efficiency.

I don't think that's the case any more, as the 12 o'clock position on my speed indicator is now more than 80 mph. Since that's illegal for sustained driving even on UK motorways, I don't do it.

I.D.10-t · Post by **I.D.10-t** » Mon Oct 13, 2008 4:20 am

I kind of wish that there was a switch to the on-board computer that would allow you to choose performance or fuel economy. I would love to have a tug-boat like car that could hall a large amount in a trailer but not be over geared and in four wheel drive all the rest of the time.

As for hypermiling, It seems like unless you drive on empty highways, that you are likely to be a road hazard for little return in fuel economy.

aderyn_du · Post by **aderyn_du** » Mon Oct 13, 2008 5:24 am

djm wrote:I find driving slow causes me to suffer far more than the mileage does.

djm

Now this is a person after my own heart!

dwest · Post by **dwest** » Mon Oct 13, 2008 6:14 am

Hyper-inflating is better, actually just keeping it slightly on the high side of the range. Coordinated stoplights are better, most localities are abject failures with that one. My city is twenty two miles long, with only two arteries that are bisected by a railroad the length of the city. It is nearly impossible to drive more than a block without getting stopped, all that stopping and going eats more fuel than speed. Traffic engineering here is very uninformed. We only have one three mile stretch where the lights are coordinated so that if you are going the speed limit you can travel the whole section non-stop. That is in the southern most part of the city which is only industrial, completely dead on the weekends and only busy between 2:30pm and 5:00pm. There are no consumer services in the southern section of the city except city hall, most services are in the northern part of the city. So the whole lower half of the city which is where I live has to go north for most services. I go through 10 stop lights to get to the grocery store five miles away, speed limit 35-45MPH, and rarely make it through more than one light/trip, an incredible amount of wasted energy.

Flyingcursor · Post by **Flyingcursor** » Mon Oct 13, 2008 6:26 am

Hypermiling isn't about going slower or faster but about using your speed wisely. For example if you leave a red light and are approching another read light, don't floor the pedal since you'll only have to brake in a few seconds. Basically, any time you have to brake you're wasting momentum which is a waste of gas.

Daniel_Bingamon · Post by **Daniel_Bingamon** » Sat Oct 18, 2008 5:16 am

dwest wrote:Hyper-inflating is better, actually just keeping it slightly on the high side of the range.....

Hyperinflating can be dangerous though. The Firestone tire recall was due to wreckage caused by hyperinflation in some cases. The properties of gases (such as air) expand and contract due to pressure.

http://en.wikipedia.org/wiki/Gay-Lussac%27s_Law
So once the car goes down the road and the tires are hyperinflated, the increased temperature from moving along the road makes the air in the tire warmer and inflates even more.

This also why tires read low in the cold of winter and have to be pumped up.

djm · Post by **djm** » Sat Oct 18, 2008 6:35 am

Keeping tires slightly soft on trucks gives them better traction by keeping more tire surface in direct contact with the ground at the same time.

djm

dwest · Post by **dwest** » Sat Oct 18, 2008 7:34 am

Daniel_Bingamon wrote:
dwest wrote:Hyper-inflating is better, actually just keeping it slightly on the high side of the range.....
Hyperinflating can be dangerous though. The Firestone tire recall was due to wreckage caused by hyperinflation in some cases. The properties of gases (such as air) expand and contract due to pressure.

http://en.wikipedia.org/wiki/Gay-Lussac%27s_Law
So once the car goes down the road and the tires are hyperinflated, the increased temperature from moving along the road makes the air in the tire warmer and inflates even more.

This also why tires read low in the cold of winter and have to be pumped up.

Every car has an inflation range for the specified tire type and load capacity. Tires should always be inflated to proper pressure while the tire is cool, not after driving for a mile or two. The inflation range posted on the car somewhere takes into account the increased pressure as a result of heat. Most drivers do not keep their tires inflated to manufacturers specified pressures, meaning they are most often under inflated.

Post by **Nanohedron** » Sat Oct 18, 2008 12:11 pm

s1m0n wrote:One the highway, the geatest efficiencies are likely to be by going whatever speed everyone else is, and collectively 'drafting' in one big pod.

Oh, that's what's going on. I thought everyone was just "herding". You know, like sheep. An unconscious social occasion.

Clustered together at 70mph in one-ton masses of potentially jagged glass and steel? Nope. Uh-uh. Not for me, thanks. I'd rather spend the few extra pennies.

Flyingcursor · Post by **Flyingcursor** » Wed Oct 22, 2008 12:24 pm

Nanohedron wrote:
s1m0n wrote:One the highway, the geatest efficiencies are likely to be by going whatever speed everyone else is, and collectively 'drafting' in one big pod.
Oh, that's what's going on. I thought everyone was just "herding". You know, like sheep. An unconscious social occasion.

Clustered together at 70mph in one-ton masses of potentially jagged glass and steel? Nope. Uh-uh. Not for me, thanks. I'd rather spend the few extra pennies.

Amen brother. I call it "tailgating" and I really wish it was legal to defenestrate big rocks at them.