More miles per gallon

[Gav]

I think it was the AA that said that the most fuel efficient way of driving is to accelerate as quickly as possible (without exceeding 3000 rpm) up to the speed you want to cruise at.

[SteveK]

The most efficient speed is 56mph apparantly.

Apparently, but it isn't true.

According to a "What Car" test of 5 cars, the slower you go in top gear, the better your fuel consumption.

Of course it isn't going to be true - cars have different gear ratios/engine characteristics. My understanding is that cars have a top gear ratio slightly lower than preferable, as most manufacturers are more bothered about the fuel economy results from standard tests than the actual driving people lots of people do (especially if they do lots of long distance motorway touring).

[Gadget]

Just a brief cautionary tale: it makes a difference who's pump you fill up at...

I was at 55.7MPG, I had a voucher for 5p of a litre at tescos so I filled it up. My estimated distance to empty: 660 miles. I spent £55 and I saved £2.50 due to the voucher.

I am now dry again, my mileage is down to 53.4MPG and I've travelled 600* miles. My driving hasn't changed, I checked my tyre pressure and I am carrying nothing additional in the car...
So I can only think on three things that would make the MPG drop so much: 1) it's colder. 2) I'm carrying the extra weight of additional fuel (I'm normally on about 1/4 tank ) or 3) Tesco dilute their fuel. Or all of the above.

Normally I use Shell - from this result, I've saved £2.50 and wasted £6.50; I would say that the odd penny of difference between rival petrol stations does not make as much difference as the fuel you're putting into your car.

(* I'm deducting the "miles to dry" before filling up from these figures so that it's a bit more accurate)

[DavidY]

1) it's colder.

It has suddenly got colder in the last week or two (here in the English Midlands anyway - dunno what it's like where you are).

It does seem to make a difference - my fuel consumption always gets a few mpg worse during the winter.

[Mr Darcy]

Apparantly diesel weighs about 0.87kg per litre, so an extra 3/4 of a tank is a significant extra weight to move. Also the colder weather means it takes longer for the engine to warm up which will use more fuel, especially on short journeys. Being colder means the air is more dense so you actually get slightly more air/fuel mixture in the engine cylinders (which is why turbo cars have intercoolers), that might make a slight difference, though probably just making it go better!

[Gadget]

Apparantly diesel weighs about 0.87kg per litre, so an extra 3/4 of a tank is a significant extra weight to move.

There is some "fuel economy wisdom" bantered about that you should actually keep the tank as full as poss: since evaporation can account for some fuel 'wastage', if there is less space for the fuel to evaporate into, then less can evaporate and so you waste less... personally I think that carrying the additional weight must increase fuel consumption more than the amount that evaporates... but that's based on nothing more than pseudo-science and a hunch.

[Mr Darcy]

I wouldn't like to say it's the case for all cars, but certainly the last few I've had (VW & Audi) have had an evaporative emissions system built in. The fuel tank is completely sealed and any vapours are stored in an activated carbon filled canister in the engine compartment. When the engine is next started the ecu controls a valve to allow the vapours to be drawn into intake manifold so that fuel is actually burnt and usefully used rather then being allowed to escape into the atmosphere. So I'd agree, carrying the weight is going to be the more significant issue.

[David Bailey]

It has suddenly got colder in the last week or two (here in the English Midlands anyway - dunno what it's like where you are).

It does seem to make a difference - my fuel consumption always gets a few mpg worse during the winter.

Hmmm, yes, me too - I've noticed it in the past couple of weeks, and I haven't changed my driving or filling-up patterns.

I reckon I got about 5-10% less mileage in the past couple of fill-ups, I was wondering why that might be.

I don't know if cold weather per se affects things, but obviously I now use headlights and heater more, so that's an increased load on the alternator, which I assume uses more energy (ie. petrol)?