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Glom
2005-Jun-08, 03:28 PM
Is fuel economy more dependent on power applied or RPM? Normally on the London-Birmingham journey, I maintain constant power and allow the speed to vary according to the gradient. However, today, I tried to maintain a constant speed of around 78mph and adjust the power to accomplish this. The economy looked significantly improved for the first half of the journey but then collapsed leading to no overall improvement. There are other factors such as the atmosphere being hot and high pressure, meaning that economy was weakened since the air was thick, leading to more drag, and the air was hot, hindering engine performance due to thermodynamic efficiency. The urban journey north side of motorway sucked because it was home time and I got caught by virtually every traffic light (fuel quantity plummetted during this terminal phase).

Captain Kidd
2005-Jun-08, 03:50 PM
This is more on you comment about the last part. How hard are you on the pedal? Stomp gas, stomp brake, repeat? Easing out of lights might get you glared at, but you're not sucking the fuel down either. I've got friends that'll keep on the gas until the last second and the brake hard at a stop and then punch it to go again. Then they wonder why they don't get the rated fuel economy.

Glom
2005-Jun-08, 04:00 PM
This is more on you comment about the last part. How hard are you on the pedal? Stomp gas, stomp brake, repeat? Easing out of lights might get you glared at, but you're not sucking the fuel down either. I've got friends that'll keep on the gas until the last second and the brake hard at a stop and then punch it to go again. Then they wonder why they don't get the rated fuel economy.

I don't drive like that.

Grey
2005-Jun-08, 04:01 PM
Is fuel economy more dependent on power applied or RPM? Normally on the London-Birmingham journey, I maintain constant power and allow the speed to vary according to the gradient.
I was told years ago when learning to drive that this is the most efficient choice. However, I'm unaware of a concrete study of the matter.

Bob
2005-Jun-08, 04:04 PM
If you wish to drive 78 mph you are not serious about fuel ecionomy.

tlbs101
2005-Jun-08, 04:32 PM
Back in the 80's I took a driving course who's emphasis was driving for efficiency.

We were taught to accelerate "briskly" to full speed, then ease off the gas pedal. After attaining the speed limit (for the traffic conditions or the posted speed in light traffic) maintain speed as the inclines demand. Do not use "harsh" movement on the gas pedal. Never "floor" it. Anticipate stops well in advance so you can idle the engine and coast to the stop, or at least coast to a point where braking is not "harsh".

Back then the speed limit in the US was 88 km/hr, and the instructor claimed that cars were less fuel efficient at higher speeds. I think that was more propaganda (this was a US department of Energy sponsored driving course). Drag does go up as the 3rd power of velocity, but there are other factors to consider.

I have found that my Toyota Corolla gets the best fuel economy around 70 to 75 mph and drops off very sharply above 75 mph.

As a side note -- I am not in any way opposed to big SUV's or other big gas-guzzlers. In fact I would love to own a Hummer. But right now, I'm sure glad I drive a car that gets 35 mpg commuting in the city and over 40 mpg on long trips across country.

Glom
2005-Jun-08, 04:42 PM
If you wish to drive 78 mph you are not serious about fuel ecionomy.

I'm also serious about not taking too long and getting tired and becoming sloppy.

Bob
2005-Jun-08, 04:46 PM
You can't have it both ways.

http://auto.howstuffworks.com/question477.htm

enginelessjohn
2005-Jun-08, 04:48 PM
Allowing speed to bleed off slightly on hills is a bit more economic I believe, as you aren't accelerating which is what consumes fuel. Additional things to consider are keeping the engine at its most efficient speed, usually around 2000 to 2500 rpm. Slow urban driving is also fuel hungry, as is cold weather. Don't forget having your tyres at the correct pressure as well.

However the primary thing to consider is drag due to speed. Drag increases at the square of the speed, and that really starts to have a significant effect above 60 mph. Keeping under that speed will help.

As an anecdotal aside, I used to commute 30 miles down country roads running a steady 50 to 60mph, and fuel consumption, on my geriatric Saab was around 35 mpg. My commute now is 10 miles half though town, and half on fast dual carriageway, and its about 30mpg. UK gallons of course. Same car, same driving style (moderately brisk.... :) ). And of course during the fuel protests a few years back, I got an extra 25% out of a tank of petrol by coasting in neutral down hills, never touching the brakes and never exceeding 50 mph....

Cheers
John

Glom
2005-Jun-08, 04:51 PM
You can't have it both ways.

http://auto.howstuffworks.com/question477.htm

I want to optimise economy at that speed. If the drive takes too long, concentration falls and safety is jeopardised. I won't care about the environment if I'm dead.

Glom
2005-Jun-08, 04:58 PM
As an anecdotal aside, I used to commute 30 miles down country roads running a steady 50 to 60mph, and fuel consumption, on my geriatric Saab was around 35 mpg.

My Nissan Micra is very economical, and I'm very proud of it for being such a thing (plus it's the last production run of the Micra before they brought out the new disgusting one). It does 45mpg in town and about 38mpg on the motorway at around 80mph. Since it's only a 1l engine, the high RPM required for high speed driving means a loss of efficiency, compared to urban driving when the thing just rolls around town unpowered. Despite the low power, it is very light and responsive. A couple of months ago, we were visiting American cousins and the reaction when they heard the economy of my car was priceless.

The weird thing I've observed is that economy improves when it's heavily loaded.

Captain Kidd
2005-Jun-08, 05:01 PM
You can't have it both ways.

http://auto.howstuffworks.com/question477.htm

I want to optimise economy at that speed. If the drive takes too long, concentration falls and safety is jeopardised. I won't care about the environment if I'm dead.

Just how long/far are you driving and how often?

Weird Dave
2005-Jun-08, 05:02 PM
Allowing speed to bleed off slightly on hills is a bit more economic I believe, as you aren't accelerating which is what consumes fuel. Additional things to consider are keeping the engine at its most efficient speed, usually around 2000 to 2500 rpm. Slow urban driving is also fuel hungry, as is cold weather. Don't forget having your tyres at the correct pressure as well.

However the primary thing to consider is drag due to speed. Drag increases at the square of the speed, and that really starts to have a significant effect above 60 mph. Keeping under that speed will help.

As an anecdotal aside, I used to commute 30 miles down country roads running a steady 50 to 60mph, and fuel consumption, on my geriatric Saab was around 35 mpg. My commute now is 10 miles half though town, and half on fast dual carriageway, and its about 30mpg. UK gallons of course. Same car, same driving style (moderately brisk.... :) ). And of course during the fuel protests a few years back, I got an extra 25% out of a tank of petrol by coasting in neutral down hills, never touching the brakes and never exceeding 50 mph....

Cheers
John

I'd have thought you get incredibly good miles-per-gallon, what with having no engine...

teddyv
2005-Jun-08, 05:06 PM
A little off topic, but another factor is tire pressure. Underinflated tires apparently can create a very significant factor, but I don't have any hard numbers.

I recall hearing an anecdote that when the US changed the highway speeds down to 55MPH for fuel conservation, they would have realized the same or better benefits by ensuring car tires were properly inflated.

ToSeek
2005-Jun-08, 05:42 PM
My wife has a Honda Insight gas-electric hybrid that shows the instantaneous gas mileage. If I were looking for pure mpg, I'd take the car at 40 mph up the hills and 80 down the hills. Obviously, that's not practical, though....

The gearing is set very much for good mileage, with very high fourth and fifth gears that aren't much use for accelerating. (Her third gear seems to be about the same as my fifth gear.)

Russ
2005-Jun-08, 06:06 PM
The secret is smoothness. Drive like you have a raw egg taped to the gas and break peddles. The object of the exercise is to not break the eggs.

Imagine another egg on your dash. If you turn, accelerat or break hard enough to crack it, you're not being fuel efficient. ;)

Bob
2005-Jun-08, 06:12 PM
I won't care about the environment if I'm dead.

If you drive 78 mph you don't care about fuel economy or safety.

SeanF
2005-Jun-08, 06:18 PM
I won't care about the environment if I'm dead.

If you drive 78 mph you don't care about fuel economy or safety.
:o

I drive right about 78 mph on the way to and from work every day.

I've never tried checking my gas mileage compared to driving at a slower speed, but I don't think I'm in significantly greater danger . . .

Captain Kidd
2005-Jun-08, 06:34 PM
I won't care about the environment if I'm dead.

If you drive 78 mph you don't care about fuel economy or safety.
I don't know about fuel economy but if it's like here, doing less than that is a safety hazard as you're likely to get rear ended or ran off the road.

tmosher
2005-Jun-08, 06:59 PM
I won't care about the environment if I'm dead.

If you drive 78 mph you don't care about fuel economy or safety.
I don't know about fuel economy but if it's like here, doing less than that is a safety hazard as you're likely to get rear ended or ran off the road.

Less than 75 mph on I-30 east results in a semi-truck on your butt.

Glom
2005-Jun-08, 07:02 PM
If you drive 78 mph you don't care about fuel economy or safety.

Driving at a different speed from the rest of the traffic is not caring about safety.