Forget MPG, think GPM

The highly respected American academic journal ‘Science’ has published a report that claims that motor manufactures have got it all wrong when it comes to the way they promote how far a vehicle will travel compared to the amount of fuel it uses.

The traditional method is to quote Miles Per Gallon (MPG) but the new report from scientists at North Carolina’s Duke University says that the traditional method needs to be turned on its head to encourage fuel saving.

The report entitled ‘The MPG Illusion’ says that manufacturers should be promoting Gallons Per Mile (GPM) to help potential buyers to choose the most fuel efficient vehicles available.

Jack Soll and Richard Larrick, the report’s authors, claim that the traditional MPG does not give buyers the correct mental image of how much fuel a vehicle uses.

They claim that if the big motor corporations were to promote fuel efficiency in terms of GPM the public would be able to better understand the amount of fuel the car actually consumes and therefore how ‘green’ the vehicle would be.

The problem, according to Soll and Larrick is that people mistakenly believe that a saving of 5 MPG equates to an equivalent amount of fuel efficiency.

They say that this is not actually the case, and in fact if one car’s fuel efficiency is 5 MPG and another is 10 MPG that is more significant than saving 25MPG on a long motorway journey.

Saving just 1 MPG in the city is the equivalent of saving 17MPG on long journey motoring, when considered over a full year’s motoring.

What the scientists have done is calculate total fuel consumption over a distance of 10,000 miles in all conditions, their conclusions are that a vehicle that does 20 MPG will use five gallons to cover 100 miles while a car only capable of 10 MPG will use 10 gallons, that’s pretty straightforward.

But a very small fuel saving car that can achieve 50 MPG or 2 gallons per 100 miles, when compared to another small vehicle that gets 25MPG actually only saves two gallons, that is of course 50% less but a saving of only two gallons, where the bigger cars saving was 5 gallons.

None of this makes the bigger vehicles actually ‘save’ more fuel, because of course they are actually using more fuel in total, but it shows that MPG figures can be psychologically misleading.

What the scientists are suggesting is that is much clearer if people were to consider fuel consumption based on per 10,000 miles traveled or GPM rather that strictly how many miles a car can cover per gallon of fuel used (MPG)

Using this method a potential buyer can easily see which vehicle is more efficient in realistic terms of how much fuel it will use in a full years typical driving of 10,000 miles rather than doing impossible mental calculations of X amount of urban mileage and Y amount of motorway driving to get an idea of how much fuel the vehicle uses, this new system would give a clear indication of a year’s fuel savings.