h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

A friend of mine only ever half fills her car with fuel, on the basis that the lighter load translates to better fuel economy. I always fill my car completely full, because I'm sure the extra time spent at the fuel station could equate to better economy, by simply driving slower. However, it's not always practical to drive slower, if for example, you are in a stream of traffic.

If you wanted better economy, what would you do?

Yes, clearly the weight of fuel makes a difference to economy and we only fill up to save time and bother.

Didn't they used to refuel F1 cars during the race for this reason? - I think they banned it after a couple of pit lane fires.

No, they banned F1 refuelling to encourage the engine manufacturers to make engines more economical.

The best way of saving fuel is to change the way you drive... avoid using your brakes if possible, avoid accelerating rapidly and if you do a lot of motorway miles lower your speed (you can get truely unbelievable mileage from most vehicles if you sit in the slow lane moving at the same speed as the trucks (55-60 mph)...

Having said that, if you're already doing all this and want to save even more fuel then driving around with a car that's 20-25 kg lighter (depending on your fuel tank size) can only help!

How heavy is half a tank of fuel? You might find you can declutter your car and remove a similar amount of weight. Unwanted tools, junk, children, etc.

Filling up your tank should be more economical because it reduces the number of journeys to the petrol station, each one of which uses fuel. (Unless of course you drive past the filling station on your normal routes).

Using your brakes to slow down instead of your engine is one way of reducing the fuel you use.

But the single biggest thing you can do is to use cruise control when on long journeys. By keeping the speed constant, it saves on lots of excess fuel used while accelerating. It's theoretically possible to drive in such a way as to use even less fuel, but it's unlikely you'll be able to manage it.

I'm confused. We have one "use your brakes less" and one "use your brakes more" if I've understood the above posts correctly.

any ideas?

It's driving style. Some people accelerate and brake unnecessarily rather than drive smoothly. You might describe those as 'people who use their brakes a lot', hence to save fuel they should adapt their driving style so as not to require using the brakes so much.

But in a one-off situation, if you brake to decelerate rather than coast to a halt, the car stops quicker and hence uses less fuel.

I normally drive with the gas tank half full. That's not an economy move to get better gas mileage, though. I could declutter the car but, pack rat that I am, I go around with some towels [in case they are needed for getting out of stuck-in-snow situations], sheet sets, jumper cables, ice scrapers, etc. I'm prepared for a lot of things that might never happen, while unprepared for what probably *will* happen.

I'm confused. We have one "use your brakes less" and one "use your brakes more" if I've understood the above posts correctly.

any ideas?
ARE YOU DRIVING AUTO OR MANUAL

>>, if you brake to decelerate rather than coast to a halt, the car stops quicker and hence uses less fuel.

I'm not sure that's logical. For a start, my car displays the mpg at any given moment, and while I'm coasting, it goes to maximum, which is 99.9 (the actual figure could be even more ). So assuming you're not coasting, in order to need to brake, you must be using some fuel while the throttle is more fully open, while coasting to a halt would require less throttle overall.

>>>Using your brakes to slow down instead of your engine is one way of reducing the fuel you use.

That used to be the case years ago, but with modern engine management systems, if a car engine is being driven by the wheels (as opposed to the normal situation where the engine is driving the wheels) then the EMS actually shuts off the fuel going into the engine - cruising to a halt your actually using ZERO fuel.

(If you've got one of those fuel consumption meters on your car, try it out - set it to show instantaneous fuel consumption and when you take your foot off the accelerator while going downhill your fuel consumption will show as 0.0 mpg.)

Freewheeling in neutral uses fuel
Accelerating uses fuel
Constant pressure on the accelerator pedal uses fuel
Releasing the accelerator and cruising to a halt uses no fuel.

The other thing to think about is the typical situation where you're driving stop-start between traffic lights...

You have to use energy (fuel) to accelerate the vehicle away from the traffic lights
-- Chemical energy >> Kinetic Energy
The ideal situation is that you convert the absolute minimum quantity of chemical energy (fuel) into kinetic energy which is needed to get your car to the next set of lights...
If you have to use your brakes to stop yourself at the next set of lights then you've used up too much chemical energy and you've got to get rid of the excess kinetic energy braking
-- Kinetic energy >> Heat Energy etc.

Slight edit to my last post...

"Quotes" is right - while coasting in gear your fuel consumption meter will read 99.9 mpg (in reality it should read "infinity" mpg)...

I've been on the continent too long and am used to looking at fuel consumption figures in litres per 100 km... which is the exact opposite of MPG - the lower the figure the better the fuel consumption.

Have they banned refueling in F1 then? Shows how long it is since i've bothered watching

Whats F1

My car has a 60 litre fuel tank. Assuming a density of 800 g per litre (about right for a hydrocarbon) if I only half fill it I save 24 kg of weight each time compared to completely filling it. That's actually quite a lot. If you've got 24 kg of clutter in your car you are doing well.

What's F1

I bought some F1 petunias last week. I'm confuzzled

20kg (i.e half a tank in my car) sounds a lot if you have to lift it by hand... but I weigh 85. I am usually carrying two paragliders in my car just in case it's flyable, for a total of about 35kg. And my car weighs over a tonne. So the 20kg is well under 2% of the total weight, so if the weight to mpg ratio is linear, I can save UP TO one mile per gallon on fuel weight. Whoopy f$%^in doo. In the context of normal driving, one single solitary mpg here or there doesn't show against the noise in the signal.

I save far more by using cruise control (which can get my fuel consumption to 50mpg on unleaded, maybe better), or if I can be bothered to concentrate, simply driving carefully, when I can get it up to more like 55mpg.

thanks for the explanations, nearly none of which my totally non-mathematical or non-engineering brain can cope with

Should we also consider the price at the time you fill up? I tend to use a tank a week (45litres of diesel) which gets me to and from work 5 times (round trip: 150kms) plus about 55kms more of willy nilly driving (taxi service for The Gruesome Twosome).

Diesel prices at the moment go from something in the region of EUR 1,49.9 to EUR 1,33.9

If I fill up on a Thursday afternoon it's usually the cheapest time of the week (however, I do notice that I get more kpg from Shell, who give me a 1ct discount per litre for being in the ADAC, rather than JET who are usually displaying a price 1ct cheaper than Shell. Although now and again I do put Tiger in my tank when that is cheaper - or I'm running low)

In theory, then, I could save more by topping up my tank whenever the price dips below, say, EUR 1,43? and not speeding along the autobahn at breakneck speed (haha - it's a rubbishy Golf Plus and I'm lucky if I get 150kph)

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