Why are your correspondents surprised that they can't match their car's official mpg figures?

Your letter-writers often complain that they can't match their car's official EC mpg figures. Surely it's common knowledge (or ought to be?) that these are measured while the car is driven in accordance with some sort of notional ‘typical' driving pattern on rollers in a laboratory, and, as the UK and other countries tax cars on this basis, manufacturers will pull every trick in the book to get good test results, even at the expense of poorer real-world economy. There will be a broad correlation between the official figures and reality, but a close match is probably coincidence.

Asked on 3 April 2011 by CD, Essex

Answered by Honest John

Since cars are taxed on their CO2, and since from 2013 manufacturers will be fined if their corporate average CO2 exceeds 130g/km, it is only natural that they optimise their cars for these tests and that performance in them bears only a mild relationship to what drivers can expect in real life. The tests are conducted in ambient temperatures of 20 – 30 degrees centigrade. So obviously if the car is heat soaked overnight in a 30 degree lab it won’t ever need to go through the fuel-hungry cold start cycle.

To inject some reality, we now offer the Real Life Fuel Economy Register (www.honestjohn.co.uk/realmpg) where drivers can regularly log their real life fuel economy. Each time they do, the average for their make, model engine and transmission is adjusted and a figure is constantly displayed giving anyone a real life average fuel economy figure.

Tags: european law fuel economy real mpg

Similar questions

How do manufacturers calculate their official fuel consumption figures?

With regard to manufacturers' fuel consumption figures for their vehicles, I fully agree with comments made in your columns about them being unachievable. However, where I feel they are of value is in...

Should my new Ford EcoBoost be returning just 25mpg?

Your ‘real mpg’ figures are hugely different from what Ford says and they never told me about the EcoBoost engine taking thousands of miles before it starts to give good economy. I do lots of motorway...

Why has Audi released inaccurate mpg figures in its brochures?

I have sent you a copy of the Audi A4 2012 brochure. In it the EC figures show the claimed improvements in mpg between the 2008 and 2012 models. In a recent County Court judgement against Audi relating...