[lclarsen]

So I know the best speed for fuel efficiency (in most cars) is around 55mph... but that's on flat roads, right?

I have to drive uphill, sometimes pretty steeply, for about 30 minutes every day. What speed should I go to get the best fuel effiency?

Thanks in advance!

(PS - I drive a 92 Honda Civic.)

[fonzcad3]

There really is no way to tell unless you know the grade that you are going up and the engine power to fuel consumption curve for your engine. Generally engines have a sweet spot based upon the engine power curve.


What you could do is figure out the power to move your civic uphill and then equate that to a power of 55MPH on flat road, you would need the grade of your hill. Essentially the steeper the hill the slower the speed.

[tsakach]

Shed as much weight as possible. I would even consider removing your spare tire, traveling on less than a full tank, removing unneeded seats, etc.

[chakra]

If you drive a standard (like I do), you can reverse a lot of that fuel usage on your way down the hill. What I do is I just push in the clutch going down hills and the motor will idle at only 1000 rpm usually and you'll still maintain lots of speed. I suppose you could use neutral in an automatic, but it's more natural in a standard.

[Madpaddy]

Don't keep the clutch pressed in chakra or you'll wear it out. Put the car int neutral and control it with the brakes (this of course will wear out the brakes quicker).

[Andy]

If the hill is steep enough and speed limit is relatively low, coast the car in gear (for manual trasmissions). I know my Jetta TDI consumes no fuel when coasting in gear, even less than idling and if the right gear is selected engine drag requires no or minimal brake use. Coasting in gear by the way is leaving the car in gear while the accelerator is not being depressed and the brake is only slighly being pressed if at all.

[yesplease]

Pick whatever speed you want to go uphill, and choose a gear that will allow you to give it lots of throttle (but not WOT), which means fairly high engine efficiency. Easy in a stick, a PITA in an auto. Generally speaking, the slower you go in top gear, the better your mileage. There will occasionally be plateaus or drops, but as a trend this is what I've observed. I would go look for LRR (low rolling resistance) tires before taking any weight out of the car.

[PraiseDoom]

$this->bbcode_second_pass_quote('tsakach', 'S')hed as much weight as possible. I would even consider removing your spare tire, traveling on less than a full tank, removing unneeded seats, etc.

[EndOfGrowth]

Give your car a good service and tune, try some high performance spark plugs and maybe treat your engine with an anti-friction/wear additive like slick 50, check your tyre pressures (inflate to max safe pressure) and stay in a lowish gear and crawl. Also check that no brakes or wheel bearings are binding. Every little helps.

[dbruning]

$this->bbcode_second_pass_quote('', '1') of every 4 lugnuts

[WisJim]

Somewhere in the 25 to 30mph range is probably where you get best fuel efficiency, not 55mph, but it varies with the kind of car, gearing, engine, etc. Probably the speed you travel in high gear at the very lowest throttle setting without the car bucking or jerking, basically idling in high gear. Of course this is just on flat smooth surfaces.

[JoeW]

The key is to try to hit the hill with as much momentum as possible, and then keep your throttle about half to three quarters. for ultimate fuel economy, you want to have exhausted your momentum by the time you reach the top. If the hill continues for miles, I would seek the advice of an expert hypermiler, but I think try to keep the rpm's around 2000 or so.

As far as coasting down hills, I don't understand why someone recommends leaving the car in gear. All that does is uses the force of gravity to turn your engine over. Neutral is the answer. I believe the most fearless hypermilers would even turn the ignition off while coasting, although I do NOT recommend you do that at home.

[oiless]

Internal combustion engines almost always achieve their best efficiency (lb/hp/hour) at their peak torque. (Please think about this and realize that best efficiency in pounds/horsepower/hour is not the same as best mpg.)
We rarely drive them that way, because we rarely need the power that peak torque gives us. For instance my engine produces 200lb/ft of torque at 2000 rpm, and 120 hp at 2400 rpm. I need about 15-20 hp to propel me down the highway at 55 mph, so I don't need that power. The most efficient way I can use the engine, which is overpowered for what it's doing (in this case) is to turn it as slowly as possible. So 1600 rpm in overdrive is the best I can do.
On a hill I can use quite a bit more of the engine's potential, but the formula is still to keep it turning as slowly as possible, given the speed you want to maintain. The internal combustion engine is a constant displacement air pump. Each cylinder pumps a given volume of air. Fuel is mixed with the air at given ratios. The slower the engine turns the less air it pumps and the less fuel it uses. Your engine uses the same amount of fuel at xxxx rpm whether it is in 1st, 2nd, 3rd, 4th, or 5th gear.
So, the highest gear you can use is the right one, with a caveat: lugging engines isn't good for them, and it isn't good for fuel efficiency. My engine is an old school slow turning, long stroke, in-line six. It loves low rpm. The short stroke 4cyl in a honda won't like low rpm so much.
About pushing the clutch in, neutral, etc., don't bother. If the throttle is closed the car won't use any more gas revved up coming down hill than at an idle. The only place it might help is if you are coming down a very long gradual incline where there is too much engine braking in even the highest gear, and you cannot maintain your desired speed with the throttle closed.
In my jurisdiction (and most others that I know of) this practice is illegal in any case.
My two bits worth.

[steam_cannon]

$this->bbcode_second_pass_quote('lclarsen', 'I') have to drive uphill, sometimes pretty steeply, for about 30 minutes every day...
...to get the best fuel efficiency?

[EndOfGrowth]

[quote= The most efficient way I can use the engine, which is overpowered for what it's doing (in this case) is to turn it as slowly as possible. So 1600 rpm in overdrive is the best I can do.
On a hill I can use quite a bit more of the engine's potential, but the formula is still to keep it turning as slowly as possible, given the speed you want to maintain. The internal combustion engine is a constant displacement air pump. Each cylinder pumps a given volume of air. Fuel is mixed with the air at given ratios. The slower the engine turns the less air it pumps and the less fuel it uses. Your engine uses the same amount of fuel at xxxx rpm whether it is in 1st, 2nd, 3rd, 4th, or 5th gear.
So, the highest gear you can use is the right one,
[/quote]

I'm not sure this is accurate, oiless. The wider the carb/injection pump is opened up, the more fuel you are using, regardless of rpm. Just like a tap that's opened up full will increase the pressure and volume of water at the end of a hose. So driving uphill in fifth gear with your foot buried in the floor is not the most econmical way to drive.

[yesplease]

$this->bbcode_second_pass_quote('EndOfGrowth', 'I')'m not sure this is accurate, oiless. The wider the carb/injection pump is opened up, the more fuel you are using, just like a tap that's opened up full will increase the pressure and volume of water at end of a hose. So driving uphill in fifth gear with your foot buried in the floor is not the most econmical way to drive.

[WisJim]

$this->bbcode_second_pass_quote('EndOfGrowth', ' ')
The wider the carb/injection pump is opened up, the more fuel you are using, regardless of rpm.

[oiless]

$this->bbcode_second_pass_quote('EndOfGrowth', '
')I'm not sure this is accurate, oiless. The wider the carb/injection pump is opened up, the more fuel you are using, regardless of rpm. Just like a tap that's opened up full will increase the pressure and volume of water at the end of a hose. So driving uphill in fifth gear with your foot buried in the floor is not the most econmical way to drive.

[lclarsen]

Thanks for the good information everyone. Some of it was of course contradictory, but I think I figured out enough. I will try and put in effect until I can change my economic need to drive uphill... hopefully soon!

[oiless]

I should add that if you want to get good mileage from a gasoline engine, (won't work on a diesel) adding a vacuum gauge to your instruments will be very useful.
You want to maintain high manifold vacuum.

For ordinary driving planning your every move on the road is needed. You want to be coasting up to red lights in gear, using engine braking, rather than driving up to them and applying the brakes. You want to minimize starting and stopping. Even rolling very slowly is easier to start from than a dead stop.

Good mileage is easier to get with a standard than an automatic, IF the driver is trying to get good mileage and knows what he/she is doing.

A car will burn exactly the same amount of fuel going down a hill in gear with your foot off the gas as it will sitting idling, no matter what the engine rpm.

Keep in mind that your engine is an air pump. Your carburation/fuel injection mixes fuel with the air the engine is pumping at a pretty exact ratio. The less air you can get your engine to pump while still getting you where you need to go the less fuel it is burning.

Not all cars are the same. Performance cars in particular often have induction and exhaust systems that are designed to work best at certain rpm ranges, this combined with good aerodynamics can give you some uncanny results.
For instance: My jeep, aerodynamics of a brick, very primitive in-line six, gets it's best mileage at 80 kph (about 50 mph).
My spouse's car, toyota supra turbo, with decent aerodynamics and a pretty efficiently designed in-line six; I don't know for sure where it get's it's best mileage. I know that it's terrible around town, maybe 20 mpg. On the highway at 100 kph (62 mph) it's better, around 25-27 mpg. However I did get 33 mpg on one tank one time. I was driving between 120 and 160 kph for most of that tank of gas. So it seems it gets better mileage at high speed than at a legal speed. Who knows what speed is most economical for that car. All I know is that it's some speed it can't legally be driven at.