What determines fuel consumption?

I've always been told that if you want to save as much fuel as possible then you should always drive in the highest gear you can, which is understandable as you can significantly reduce the amount of revs your engine is doing my changing up.

So I know that the number of revs affects fuel consumption, as every revolution takes a small amount of fuel. But what else affects how much fuel your engine is using, rather than just the number of revolutions per minute you happen to be doing?

Reply to
Mark Hewitt
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Load on the engine, load on the alternator from electrical systems in use (aircon/lights heating/cooling fans), drag on the car though aerodynamics, changes in drag through having windows open, steady cruise compared to stop start.

all have an affect on fuel consumption.

My Celica has popup lights. Dark nights with a combination of power use through the lights being in use, and change in aerodynamics due to them being popped up and changing the profile of the car have cost me 3mpg last week.

Reply to
Sleeker GT Phwoar

The message from "Mark Hewitt" contains these words:

High gear is not /always/ the best. Often - but not always. Engines have a rev range over which they're most efficient - stay within this.

Other things that affect fuel consumption:

More than anything else, speed. Most cars have a speed at which they are particularly efficient, and for historical reasons connected to the requirement to publish mpg figures this is often around 56mph/90kph.

Driving style. I can vary the fuel consumption of my car by around 40% over the same journey just by being sensible. Accelerate smoothly, predict the traffic patterns so you needn't slow down more than necessary and avoid complete stops almost entirely.

Loading. A heavily loaded car will use more fuel.

Tyre pressures. Under inflated tyres drink fuel. As do badly balanced tyres.

Maintenance. Not as much of an issue now that properly metered fuel injection is common, but if the engine management isn't doing its job properly you'll pay.

Roof racks, roof bars and open windows. All these use quite a lot of fuel at speed. The wife's Clio handbook reckons 10% increase for windows open at 90kph. Or something like that. I CBA to get up and find the book.

Reply to
Guy King

My 90 Prelude used to do 5mpg less on average over the winter, at least in part because I had to have the headlights flipped up most of the time. If it was really cold I would leave them up (but not on) even in the day, because otherwise they might freeze shut!

Al

Reply to
Al Reynolds

Lowest fuel consumption is normally achieved when driving steadily along on a level road in top gear at the lowest speed without slogging the engine. All auxiliaries need to be switched off, both mechanical such as aircon and electrical. The engine needs to be at normal working temperature. All windows need to be closed and no 'extras' added like roof racks or additional lights to spoil the aerodynamics.

Change any of the above and fuel consumption will increase unless you drive down a steep hill with a following wind, which you can't do for long.

Ian

Reply to
Ian

giving it welly on acceleration is a big one. Try to put your foot down as little as possible.

Reply to
jeremy

My wife is amazed that I can get 15% more out of her 1.4 Clio than she can ,when I often use the available performance more than her (for overtaking and the like).

It's because of what Guy says here. She always wooshes off from cold, leaves braking far too late, and seems to think you can't use fifth gear below

45mph. In other words - she's hardly ever in the appropriate gear, and stops more often.
Reply to
DocDelete

Load on the engine = load on the alternator from electrical systems in use (aircon/lights heating/cooling fans), drag on the car though aerodynamics, changes in drag through having windows open

You could have stopped after the first comma

Reply to
Marc

Bollocks!

Reply to
Marc

From the engine's point of view, it's how much fuel gets squirted in for each bang. This is related to throttle/accelerator position - press down on the pedal and you'll get more fuel in.

As everybody else has pointed out, put more load on the engine and you'll use more fuel. The way this works is you'll actually be pressing the pedal a bit more than before - even if it's only a little bit. If for example you were to compare 4th and 3rd gear at 30mph, you might find the fuel consumption is similar, or possibly even lower for the 3rd gear case. Obviously the revs are different, but so is how far you've pressed the pedal - less fuel per bang for the 3rd gear case.

There are other complications, such a giving more fuel while accelerating and automatic idle, but the basic thing is that more load = more pedal = more fuel.

cheers, clive

Reply to
Clive George

Not all load on engine is electrical load though. Which is what I was refering to, it could be because the car is towing (you could argue that is aerodynamics) or loaded upto to the roof lining with cases of newcastle brown ale.

Reply to
Sleeker GT Phwoar

Nope, not at all. Manufacturers could slipstream wagons to get their based on fiction combined figures by pushing up the motorway figure.

Reply to
Sleeker GT Phwoar

There is a nice graph (well, several graphs) in Figure 5 at

Fuel consumption is supposed to be minimised at 65-70 kph (40-42mph) and has increased by about 10% at 45kph and 90kph (30mph and 56mph)

Reply to
Nick Finnigan

There's also the small matter of the energy required in getting (and keeping) the extra mass of the trailer moving.

More likely to be Kronenbourg and half a ton of rolling tobacco "for personal use, honest"

Cheers,

Reply to
James Dore

Whilst that is true, he illustrated the different factors which make up the load on the engine. Perhaps he didn't make it clear but still at least he gave a more interesting answer.

Reply to
neutron

It's not bollocks, cars do have a speed they are most efficient at. it varies, and I doubt it is 56mph for most cars but 56 is a fairly efficient speed to travel at. It had always been measured at 56mph for historical reasons.

It would be interesting if manufacturers published the speed and gear at which their cars are most efficient.

Reply to
neutron

I disagree with this. It isnt always the lowest speed in top gear without straining the engine which is most efficient. It can be a bit faster I think. It depends on the engine's characteristics and the gearing. I wish manufacturers would say what the most efficient speed is for their cars.

I remember when my dad had a Maestro diesel, the brochure said it did 90mpg at 30mph. You couldn't do 30mph in 5th gear so I can only assume it was most efficient in 4th gear.

Reply to
neutron

message

Eh, you press the pedal further in 3rd gear.

Reply to
neutron

I thought the main reason was that it is equal to 90km/h, same for 75mph - it's exactly equal to 120km/h.

Well probably not exactly, but 56 and 75 are the equivalents of 90 and

120kph, rounded to the nearest whole mile per hour.

Peter

-- "Diamonds are what I really need - think I'll rob a store, escape the law, and live in Italy. Lately, my luck has been so bad, you know the roulette wheel, it's a crooked deal, I'm losing all I had."

Reply to
AstraVanMan

Depends on gearing - if 4th is a lot higher the engine may struggle to keep the speed up at low rpm so you need to press the pedal further. Similar when accelerating - to speed up in 5th from a low speed you'll probably need to floor it, so you are dumping loads of fuel in and slowly getting quicker. In

3rd, say, you might only have to touch the pedal lightly to speed up as the gearing helps, so you are using less fuel even though the rpm's are higher.
Reply to
James

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