Everyone who keeps a keen eye on their fuel economy will have their own way of checking their figures. Long term, the most accurate way is to log all fuel purchases and the mileage covered, and then calculate mpg by dividing the miles you’ve driven by the number of gallons you filled up with, maybe over a full period of ownership, or over periods within that longer term. Any inaccuracies in topping up the tank will gradually shrink and you can easily draw a line through the ups and downs of your graph that will identify any trends.
But many drivers will be happy using the car’s on-board-computer or driver information system to keep any eye on fuel economy; that’s fine for that particular car, if not for making comparisons with other cars, as these systems are typically only plus or minus five per cent accurate. That’s no real handicap in monitoring variations in your day-to-day fuel economy, and usually you can draw on figures for any particular journey as it progresses, and sometimes for each tankful, or you can reset one of the displays each time you refuel, if your car has two trip gauges. Many offer an “instantaneous” mpg figure that runs in sympathy with your accelerator pedal and the gears selected, but a display that resets after the car has been stopped for over two hours is probably most useful, particularly for observing how cold starts affect fuel economy.
There’s no doubt that what you do in the first two or three minutes after start-up can either give you a good start for a fairly economical journey, or alternatively make it an expensive trip to see Uncle Jim living 30 miles away, with much the same applying for the return trip, if you’ve stopped for more than a couple of hours and the engine is cold again when you depart. Test it and see for yourself, if you don’t believe us! If you start your car up and zero the trip mpg display before your wife and the kids are safely on board and all strapped in, and if you linger at the roadside while your wife closes the gate, or goes back into the house to check that the iron is unplugged, or the alarm is set, half a mile down the road a nasty figure of something like 8.2 or 11.3mpg will be showing! Factor in a junction or two, a couple of sets of red traffic lights, and two busy roundabouts before you clear town and head into the country, and you’ll be lucky by then to have covered two miles at something like 16mpg. Not a good start! If you then drive carefully and cover the next 18 miles at 48mpg, those first two miles will have dragged your overall economy over the first 20 miles down to just 40mpg and another ten miles to Uncle Jim’s will only drag it up to 42mpg! Those incredibly thirsty two miles use as much fuel as taking you six miles during the rest of the journey, and this is by no means the worst case scenario.
Imagine you stopped after a mile or two at a supermarket to buy a bottle of something for Uncle Jim’s birthday, found the parking absolutely chocker with cars, took a full two minutes to find a space, and left the car ticking over while the wife and kids pop into the shop. That probably knocks the overall trip average over 30 miles down to around 35mpg or less. So, bearing in mind that you’ve probably covered an extra mile and a half each way diverting to the supermarket, and sat in a queue to get out, you’ll probably use a gallon of fuel for the 33 miles, when it could have been 2/3rds of a gallon for 30 miles at 45mpg. All for lack of some thought and planning, starting your engine before you needed, and not making a swift getaway. This is not just a shocking hypothetical tale, it’s real life motoring, as practised by many people who moan about fuel prices, but just don’t think about how they waste it, and it’s mostly avoidable, if you put your mind to it!