Improving Economy

Maybe this section will get read a lot, or maybe not at all, but I wouldn’t have ever got into the diesel world if I wasn’t looking for good fuel economy, well better fuel economy anyway!

How to improve fuel economy

Firstly those magic magnets don’t work. If they did car manufacturers would fit them as standard, so don’t waste your money

Ok now onto the more practical things you can do to improve ecomony.

Driving Style

The biggest thing by a big margin is driving style, and it costs you nothing to modify your driving style. I’m not suggesting we all drive like grannies out on a sunday run, but avoiding harsh acceleration and braking, and planning ahead to reduce the need to slow down can all have a big effect on the fuel bills. As an example I typically get 40-43mpg from my diesel 600. I admit I drive it quite hard and its a big heavy motor. However simply altering my driving style when I was on holiday changed the average economy and increased it to 51mpg. That is a 25% improvement which is good by anyones standards and nothing else you’ll do will come close to achieving such large improvements.

Ok so you’ve looked at your driving style. What else is there?

Fuel leaks

Well look at the obvious things first. Are there any fuel leaks? The injector leak-off pipes often weep diesel and due to the undertray you rarely see drips under the car. Remove the cover and take a look. If the pipes look wet then replace them.


Next if you have rear discs then the handbrakes are known to give trouble and so it is worth checking that they aren’t binding and that they work as expected. I recently checked my 600 to find that one wheel didn’t have any handbrake effort at all!


Whilst you are looking at the wheels there are two other important things. The first is tyre pressure. Make sure it is kept at the correct level, if the pressure is low then more effort is needed to turn the wheels which means more fuel is used. On the same subject the type of tyres you use can also have a measureable effect. Most brands will have a low fuel consumption tyre. They tyres have low rolling resistance and are made from rubber which is more resilient than normal tyres and have a different trear profile. This is good for fuel consumption but not usually good for grip. I think the first tyre manufacturer to market such tyres were michelin with their “energy” brand. These came fitted as standard to my 600, and when they were changed for michelin primacy (a more sport orientated tyre), I lost 2mpg on average, but gained more grip so like many things it is a comprimise! You’ll see that word a lot in my ramblings!


Other obvious things to maximise fuel economy are to minimise the weight you carry around. Don’t carry a boot full of junk about if you don’t have to! Then there is the problem of short start stop journies. The diesel engine is a big lump and it takes a while to get to operating temperature. If you do lots of short start stop journies then your economy will be worse than if your car spends its life cruising down the motorway.


Another less obvious things to improve ecomony is to use a lighter grade of oil, e.g. 5w40 rather than 10w40 or 15w40. I’ll not go into detail here about oil grades as it would take a LONG time! However the first number e.g. 5w is the (viscosity), thickness of the oil when it is cold. The last number e.g. 40 is the thickness of the oil when it is hot. The lower the numbers the better the economy in general which is why many modern diesels use 5w30 etc. However a word of warning, the lower the hot viscosity the lower the protection the oil provides to the engine. Its more complicated than that but I won’t go into detail here. The long and short of it is for UK temperatures a hot rating of 40 or higher is what we need. 0w40, 5w40 and 10w40 are what you are looking for in the shops for the UK climate. I’m not going to recommend a brand or a change interval as it depends on your driving. Oil should be changed at least annually as the corrosion inhibitors get used up over time. Some cars doing short start stop journies or being driven hard will need changing every 6k miles. A reps car that spends its life on the motorway might not need changing till 15-18k. This is the reason that many modern cars have gone to an “intelligent” oil service interval that is dependant on the number of cold starts etc rather than the absolute number of the miles. I change mine roughly every 12k miles which is about every 9 months.

My top tip here is to write the milage on the oil filter with a permenant marker before you fit it so that you don’t forget when the oil was last changed! (With my current 6 cars keeping track can be quite tricky!)


Other tips for maximising fuel economy is to leave the aircon off. That can add 5% to your fuel economy on some cars. Also leave the windows wound up. This reduces drag and so improves fuel consumption by roughly the same amount. Fitting big low bumpers and spoilers etc will also hurt fuel consumption but by how much will depend on what you have fitted. In general it’ll be a small effect.