Fuel economy explained

You have a vehicle. It does not have “fuel economy”.

You have fuel. It ALSO does not have fuel economy.

Put the two together and you do have fuel economy.

Did you know that electric vehicles actually pre-date petrol / gasoline vehicles? The major downside even a century later is that electric power does not have the same energy density as a gallon of fuel. Your starter battery in your vehicle, whether it is a 2 seater light weight sports car or a large 4WD truck, will be somewhere between 20 to 50 pounds in weight / 10kg to 25kg.

Put that battery into an electric vehicle and even the most modern and lightweight electric vehicle will travel no more than about 6 miles. (Modern vehicles are approaching 150Wh per mile) and that is very optimistic. Take that same SPACE occupied by the battery and a petrol / gasoline engine will travel 40 miles / 60 kilometres conservatively. Take that same WEIGHT of the battery and you will travel far further. A starter battery of 50 pounds in weight (25kg) in a modern petrol car could travel more than 300 miles!

Due to this energy density, oil based vehicles, either petrol or diesel have dominated the market. They are however not overly efficient.

The above diagram of a passenger vehicle using the American EPA Urban cycle definitions shows that only 12% of the energy from the fuel ends up driving the wheels. A massive 62% of the energy is lost as heat.

Checking fuel economy

Fill up your tank as full as possible (initial fill). If you are using a classic vehicle, avoid supermarket fuel as some owners have found reduced fuel economy and other issues. Choose the RON fuel for your engine as applicable. Note the first odometer reading.

Drive normally and at a suitable point – ideally later in the tank not sooner to reduce the error margin, fill up again (second fill). Note the second odometer reading.

For those of you in America, you just filled up in gallons. For those in Europe, if you want to stay in litres then great. To convert to UK gallons, divide the number of litres by 4.56. So 45.6 litres is 10 UK gallons. Gallons are smaller in America!

(Second odometer reading) – (First odometer reading) is the distance travelled between fills. The fuel added in the second fill is how much fuel you needed to travel.

(Distance travelled between fills) / (Second fill) = Fuel economy.

Talking about a Volkswagen transporter, the older vehicles will be towards the 15 miles per UK gallon (12 miles per US gallon) or 1.6 miles per litre. More modern vehicles can get towards 50mpg (40mpg in the US or 11 miles per litre) and custom engines can make a big dent in this figure!

Some modern calculations are litres per 100 km / 60 miles. This is also valid but for this, the lower this number, the more efficient! MPG means a higher number is better.

Next week, we will be discussing improvements in fuel economy.