Peter de Nayer tells the truth about fuel consumption

It’s an increasingly important part of anyone’s car test appraisal , but what are the rules for getting a reliable result?  Here are a few points to steer you in the right direction……

Don’t believe the Official figures; even the Urban test will be difficult to match in short journey driving from a real cold start (the official test’s cold start is between 20-30 degrees C).

Don’t rely on the car’s trip computer alone; some I have sampled were 10-12% optimistic, whereas recent Citroen C2 and Mitsubishi Colt's MPG read-outs were over 5% pessimistic.

Unless you resort to special fitment fuel metering equipment, as I do, you are going to have to buy some fuel for brim-to-brim assessment.

You must visit the same fuel dispensing pump twice, first for an initial brim and subsequently, after you have got through at least 30 litres – the more you put in between your first and last brims, the better. This may well mean returning the car with a generous fuel gauge reading. However, you will find that your PR contact will be happy to arrange that the car is delivered half full, with the driver prepared to pay the bill but let you fill it up, just before you drop him off at the station!

On this initial fill, note the exact location of the car and the dispenser pump as you will be coming back later in the test to this very same spot.  With the nozzle fully inserted, fill at full pump flow until the pump switches off, count to ten, then continue to fill with the trigger set so that the tenths of a litre are tumbling at the same rate as the whole litre tumbler was rotating before.

When the pump switches off again (at this rate it shouldn’t flood out of the filler neck – but be alert to the possibility), you are ready to roll.  As a matter of interest, note the difference between the first and second shut-off points, in litres – it’s likely to be 2 -3.

Note the odometer reading or zero the trip, before you drive away.  You will have an instrument that is normally reading high (ie long) by about 1-3%, unless you correct it. You can do this thanks to the accurate tenth-of-a-km posts on motorways, which are even numbered for your convenience; 161 equals 10 miles, as near as it matters.

You can do this once with your own car, then set up a convenient circular, anti-clockwise route near to your home, of about 10 miles, to use on all subsequent tests, as a ‘known distance’. Start and finish at exactly the same spot, to establish a precise error.

At any point later, so long as you keep a record of any intermediate litres of fuel added, you can return to your first friendly forecourt and repeat the same procedure again, noting the odometer reading on your receipt. If you have got through 30 litres of fuel over the whole exercise, you can be pretty certain that any disparity in the first and final fills will not be on the wrong side of a litre, so your MPG result will be accurate to within one or two mpg.

If you have a twin read-out trip compute, like VW Group products provide, you can correct it at the same time as the foregoing test, at one level, and take intermediate spot-readings at the other level, engaging in different types of use. Don’t be surprised when the MPG in town reads half of that recorded for gentle rural jaunts, with a dual carriageway result frequently half-way in between,  Later, you will be able to apply a correction to all trip computer readings by comparing the overall MPG reading with your calculated brim-to-brim one.

There’s a recent snag that has arisen, however.  If you are testing a diesel and it is equipped with a DPF (diesel particulate filter), it is just possible that at some point in your test it will go through a purge period. It will be undetectable to both you and the car’s trip computer but for a very short period (about five miles) consumption could increase by as much as 60 per cent! This means your brim to brim test will give a result that reflects that extra fuel consumed – so in this context, one can see that brim to brim calculations over shorter distances are nowadays out of the question.

Peter de Nayer