Improving your fuel economy
most of the info in this writeup is applicable to all cars and all countries, but keep in mind that UK gallons are not the same as US gallons (1 US gallon is 0.83 UK gallons. 1 UK gallon is 1.20 US gallons). Also, "petrol" is gasoline.
Finding your MPG
Petrol mileage is an important part of motoring: With current petrol prices, the less petrol (or diesel) your vehicle takes, the cheaper it is to run. A bad MPG may also be an indication of your engine needing a tune-up, or that something more serious is amiss.
Finding out how much fuel your car uses is simple: Put one gallon of petrol in your car, and then drive until your car stalls. Note the mileage. Voila.
Of course, there are many problems with this method: For one, the catalytic converter in many modern cars can be damaged from running out of petrol, which in turn may give you problems with emission ratings at your next MOT test. Also, being stranded at the side of the road is not recommendable. Finally, you would be hard pushed to find a petrol station in the UK that will dispense petrol in gallons, rather than in litres.
Unfortunately, petrol mileage is given in MPG - Miles per Gallon - so we will have to stick to the empirical system nonetheless.
Instead of using the example above, try the following: Completely fill up your petrol tank, note your mileage, and drive normally. When you next fill up your petrol tank, note how many litres you have put in, and how many miles you have driven. To find your MPG, just divide the number of miles by the number of gallons your vehicle has used. To convert litres to gallons, multiply the number of litres by 0.22.
For example: If you have driven 139 miles, and you have used 23 litres of fuel, you would type the following into your calculator: First we find how many gallons of fuel you are using, by multiplying 23 by 0.22. This reveals you have filled 5.06 gallons of fuel. Now, divide 139 miles by 5.06 gallons to get miles per gallon. If your car ran with these particular stats, you would be getting 27.5 miles per gallon, approximately.
Improving your fuel economy
Now that you know how much petrol your car actually takes, you can try improving it, if you want.
To save petrol, it is worth remembering what actually makes your car burn petrol. Without mixing too much physics into this, it is all about inertia: A car that is moving "wants" to continue moving. This is why motorway cruising takes relatively little petrol: maintaining speed takes a lot less petrol than gaining speed. (Braking also uses a lot of energy, in the form of brakepads and heat generation, but that has no impact on MPG, so we shall gleefully ignore that in this article).
For the same reason, city driving takes a lot of petrol. The explanation is quite simple: Starting and stopping a 1.5 tonne lump of metal takes a lot of energy. You can, however, try to help your car along, by taking it as easy as possible: Keep a lot of distance between you and the car in front. That gives you a bit of a buffer, in order to be able to break and accelerate more gently. This is useful, because a car will use less petrol when accelerating slowly (a small change in speed will take a lot less petrol than a large change in speed), than if you are burning rubber at every stoplight.
The oldest trick in the book is to imagine that you are driving with an egg between your foot and the accelerator pedal: You have to drive without breaking the egg. Slowly depress it when you need to accelerate. You'd be amazed to find out how much petrol this saves.
The next trick is to try to keep your car in its optimum power band. To find out what this is, accelerate slowly, and keep an eye on the rev-counter. Where the car seems to run with the least effort will be its "sweet spot". In most family cars with petrol engines, this will be between 2500-3000 rpm (rounds per minute). Smaller engines tend to run better at the higher end of that rev range, while larger engines and diesels tend to run better between 2000-2500 rpm.
It is very easy to become obsessive about mileage - there are actually people out there who build cars especially for extreme mileage rates, and manage to get 530 mpg, by using super-efficient, very light-weight one-person vehicles driven strategically by specially trained drivers. There is no point in going over the top, but when you think about it, fuel costs are going to be your biggest motoring cost - you may as well try and keep it as low as possible!
While we can not hope for 530 MPG results, some improvement can easily be had - if you have your car serviced regularly, and drive while paying some attention to the tips managed above, you should be able to improve your car mileage by 3 mpg on mixed driving. If you drive 15,000 miles a year, this means you save nearly 200 litres of petrol - that's £170 at today's petrol prices - just for changing your driving style ever so slightly!
Originally written for http://www.askaprice.com