Thanks for your comments. I'll ask and see if anyone at HJ would like to answer this.

I agree with your points about driving style and the type of driving done - I'd made a very similar set of observations on another thread some time ago. The Tyre Reviews website takes into account driving style, and I think HJ's should on the Real mpg page.

I also think we should have the ability, through our login, to update our own submissions, as (similar to the tyre reviews on the other website) cars change their mpg as they age, and often quite appreciably over the first year as the engine wears in (normally for the better).

Many people we see complaining about their car's mpg often do so soon after purchasing it, quite often when its new, so inputting such figures can give a false result, similarly as the new owner gets used to the car even if its not brand new.

Whilst I appreciate there are lots of variables and we may not be able to include them all to get a hyper-accurate result, perhaps the following should be at least discussed, though whether its too late after the real mpg database is now quite large, I'm not sure.

Variables for car use that affect mpg:

• Driving style;
• Length and type of journeys predominantly driven (including type of road driven on);
• Load in car (weight of driver and passengers plus boot load) normally used;
• Type of weather (incl. whether the heater or A/C is regularly used) and traffic conditions experienced when driving;
• Age of car;
• Grade of fuel used;
• Tyres used, including type/model, age and wear;
• Whether a roof rack/box or trailer is fitted/regularly used.

Many of the above might only make a small difference for most of us, but for some people, they could make, either individually or combined, huge difference - for example a retired person owning a car and occasionally doing a longer journey to friends outside the rush hour would like do a far higher mpg than a parent driving the same car with kids going to a local school and the shops a lot. I suppose at least HJ's page has a range of mpgs quoted as well as the average.

Don't worry, you won't do any harm by submitting duplicate values. We do identify duplicates and combine them before calculating the overall average.

There's a tricky balance between asking for lots of details about each person's driving or trusting that these factors will balance out in the average if we have enough submissions. The more questions we ask, the more likely it is that people don't bother to fill in the form or just enter a guess that sounds about right. There are, for example, a suspicious number of people driving 10,000 or 12,000 miles per year.

Having said that, there will certainly be models which are more likely to be driven in town and others which are more likely to spend all their time crusing on motorways, and parts of the country where people consistently get better or worse MPG. We can definitely see a seasonal trend in results. With all these factors, the challenge is really deciding how to present the data in a useful way.