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How To Find The Real MPG For A Car You Don’t Yet Own

One of the most frequent complaints drivers have is about the fuel economy their car returns. For every car, the manufacturer provides consumers with figures that allow them to see the official miles per gallon (mpg) figure that the vehicle is capable of. The trouble is, the fuel consumption drivers go on to achieve once they own the car, is usually nowhere near as impressive as this ‘official’ figure.

James Foxall
Written by James Foxall
Edited on
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But if you’re on the hunt for a new car and want to know what sort of economy you can expect over the sort of miles you cover, there is a way to find out before you spend your heard-earned money on a car that may or may not be as frugal as you’d hoped.

It’s quick and easy to do, and could save you from making a costly mistake.

True MPG on What Car?

The easiest way to find out the kind of economy you can really expect from a car is to go to the True MPG page of the What Car? website.

image of a website displaying the mpg of various cars

First you find the make, model and engine type of your car. You can then input the kind of driving you do. This includes the types of road, the levels of congestion and the sort of driver you consider yourself to be. Add in your mileage and the website calculates your ‘true’ mpg.

Actually it comes up with three figures: the ‘true’ mpg for your car; the average ‘true’ mpg for that car and the ‘official’ figure for comparison.

Does it work?

Yes and no. We put in the details and driving conditions for two very different types of car. On one, a Volkswagen Polo 1.2 TSI petrol model driven predominantly around country roads, the ‘true’ figure from the website was 45mpg, 1mpg or 2 per cent less than we actually see. In fact, our actual figure was exactly between the ‘true’ mpg and the average ‘true’ mpg. Pretty impressive.

However, for a Mazda CX-5 2.2-litre diesel automatic the results were less striking. The website claimed our ‘true’ mpg was 61mpg. That’s identical to the official figure and about 44 per cent better than the 42mpg we actually see. Strangely, the website’s average ‘true’ mpg was much closer to ours at 48mpg.

Equa Index real-world fuel economy

An independent company called Emissions Analytics specialises in harvesting what it claims are accurate mpg figures. It has come up with a site called the Equa Index. By conducting real-world driving tests on more than 800 vehicles, it says it can accurately calculate mpg for vehicles it hasn’t tested.

Equa Index Table of MPG

The downside of this website is its usability. The results of its tests are displayed in spreadsheet format ‑ with 86,854 entries! You can search by manufacturer and model so it’s worth persevering, because its testing processes are considered some of the most accurate in the car industry, and reflective of the sort of driving most of us do.

But is it accurate?

When we searched for our Polo, it came back with 43mpg, around 7 per cent less than we actually see. For our Mazda, the Equa Index claimed a 43mpg true mpg. Compared to what we’re actually getting, that’s less than 2 per cent better which is pretty impressive and a useful guide for car buyers. real mpg

Rather than using calculated figures, the website uses feedback from owners of the cars. It’s a simple concept and the website claims to have information on more than 100,000 motors. The site is also easy to use. You simply click on your manufacturer, find your model and if readers have input data on the car it will be there.

HonestJohn MPG Index Calculator

The downsides are that you’ve no idea how many people have submitted figures for your exact vehicle, how those people drive, or what kind of mileages they cover.

But is it accurate?

For our VW Polo, the readers claim we should be seeing 48mpg. That’s around 5 per cent better than we actually get.

When it came to the Mazda it was further away and the only one of these real mpg calculators to give a lower reading for this car than we’re really getting. Its 39mpg was around 7 per cent lower than our actual figure.


None of these real-world mpg calculators is perfect. But in our limited experience, the Equa Index is the most accurate, Honestjohn’s the least. Shame the Equa Index is so clunky to use.

Why don’t ‘official’ figures match these?

When cars are tested to get the fuel consumption that manufacturers use in their sales literature, they all undergo the same tests. These are done in laboratory conditions to ensure all car makers perform the same tests, thereby giving consumers a like-for-like comparison.

However, car makers employ lots of clever people and they’ve come up with ways to legally skew results for improved performance. Throw in that the official tests don’t account for gradients or wind and that the tests are done in warm conditions with no in-car equipment such as air-con or lights switched on and you have a series of tests that don’t reflect the way cars are used.