A COUPLE of people I've heard recently have said things like ‘I get about 300 miles for Â£40 worth', thinking it's a real measure of their car's economy.
But although that's a very basic guide, the only way to check the figure accurately is to fill the car up until the pump clicks off and note the mileage or set the trip to zero.
Fill it in the same way when you've done at least 100 miles and preferably more, and note the miles you've done and how many litres of fuel you add.
Convert the litres to gallons - just divide by 4.55 - and then divide the mileage by the number of gallons. That's your true miles per gallon figure.
It's worth doing occasionally because a big drop can indicate problems.
I did this - as I often do - when I drove the VW High up! - which despite its silly name is a very good city car that will carry four.
The government fuel economy figures are all a joke, as you probably know, and should only be used for comparison - the up! is supposed to do 68.9 miles to the gallon.
Suffice it to say that the best I could manage on a 400-plus mile trip, using every kind of road, was 48mpg. I usually find the real every day driving figure is about 20mpg less than the government gets.
The up! has a 1.0-litre three cylinder petrol engine producing 60, 75 or 90bhp and I drove the mid-range model in five door form.
It's immensely easy to drive in every way and can be plenty of fun thanks to excellent handling and wonderfully sticky road-holding.
It has a slick five-speed gearchange and the smooth and very quiet engine revs easily and gives decent performance.
There's enough acceleration for slingshot overtaking and I really enjoyed it on my long trip across the country.
A large part one way was done on the motorway, where it easily kept up! with the other traffic, but coming back was all on Welsh mountain and country two lane roads on a marvellously bright autumn day.
I had a great time. The ride was comfortable over most surfaces and the only thing that slowed me was queues of fairly slow moving traffic where everyone in front of me had forgotten how to overtake. It's a dying art.
Equipment in High up! spec includes stability control, brake assist, electric differential lock, hill hold, traction control, remote locking, aircon and very good stop start.
It also comes with a DAB stereo, heated front seats, electric front windows, alloys, split fold rear seat and a Bluetooth smartphone link.