Experience shows that HJ Real mpg is pretty close to the truth in about 60% of cars we have owned.

2002 Mondeo 2.0 TDCi 130 PS. Over 3 years and 36,000 miles we averaged just under 40 mpg. On a 430 mile holiday run we could just get over the official figure of 47.1 mgg, we got about 48 mpg. HJ real mpgs quote 48.8 mpg, totally unbelievable. No dash display thus no comparison.

2005 C-Max 1.6 TDCi 110 PS. Over 5 years we averaged about 46 mpg, on the holiday run it would do about 55 mpg. The official figure for the car was about 57 mpg and HJ real mpg figure is 51.8 mpg. Closer than the Mondeo but still nowhere near the truth.The dash display was pretty accurate.

HJ's figure for the BMW 118d was 48.8 mpg, Ours averaged 47.5 over 5 1/2 years and 40,000 miles but ours was the later much more economical version and there is no distinction between the earlier and later ones in the figures. As for the dash, ours said 53 mpg normally, a total lie.

The Kia Ceed 1.6 CRDi, Seat Leon 1.4 TSi 140 PS and Skoda Superb TSi 150 PS are all pretty much spot on the HJ figures. The dash in the Kia read below the calculated figure (many reported that on the Kia forum), the Superb is spot on but the Leon was almost as bad a liar as the BMW. Our Fabia 1.0 TSi 110 PS we have had about 6 months is averaging about 1 mpg better than HJ's 49.4 mpg and again the dash looks pretty much spot on.

However, our 2015 Nissan Note 1.2 DIG-S averaged about 48 mpg over the 3 years and 25,000 miles we owned it, HJ's figures suggest 44.9 mpg, a bit below what we got. Dash over read by about 1.5 mpg so not that bad.

HJ Real mpg is still my first port of call when buying a car, luckily I never consulted it before buying the Mondeo or Focus, would have been very annoyed with the differences.

The problem is that no one is the typical or average driver.

Some people spend lot of time on trunk roads or motorways at relatively high speed but also very economical. Others do little mileage and all of it around town, thus very uneconomical. I am constantly amazed at the economy figures some people acheive, but then my driving is closer to the latter than the former.

I remember one person telling me that you should take the urban figure, add 10% and assume that's the worst case. It works for me.

In any event, fuel bills are such a small proportion of the actual running cost of a car; unless of course you do huge mileages. Over say 9,000 miles, the difference in fuel uasage between a car doing say 30mpg and 40mpg is 341 litres, so about £400. Compared to the depreciation and likely enjoyment of the less economical but probably faster car, £400 is peanuts. If the difference was between 40mpg and 50mpg the money difference would be even less.

The only time to really consider the economy is when you are looking to change car and you want extreme economy, but then you simply buy the smallest, lightest car you can cope with; otherwise buy the car you can want.

Edited by Happy Blue! on 03/12/2018 at 19:32

I measure mine using fuel in and miles travelled. The in car mpg estimate is always ~5% too high.

VW Up 1.0 non turbo: HJ average 54.5 mpg, mine ~65 mpg summer, ~60 mpg winter.

Ford Ka 1.3L: HJ average 38.7 mpg, mine ~45 mpg.

VW Polo 2018 95 PS: HJ average 55 mpg, mine 64 mpg summer, 60 mpg winter.

So I get consistently 10-20% better than the average, so it's not a bad guide once you know your driving style, and the nature of your commute, and can compensate the HJ figure. It's not clear over how long a period the HJ values are for, and whether they are summer, winter or all year. I find that on a cold day (0 Celcius) mpg on a 25 mile commute plummets by 10% compared to a warmer day (10 Celcius). Also it might be that certain cars are used more on long commutes and others on city driving, which has a significant impact.

One thing I noticed is that the 95 TSI 2018 Polo gets much better mpg than the non turbo version, according to HJ, although we don't know the sample sizes. Perhaps the non turbo is thrashed more, certainly one I borrowed for a morning was overtaken by snails hurtling past.

I can average sub 30mpg in cold weather in very hilly country if there is a lot of traffic (single track with passing spaces). Even in summer it is sub 40 mpg.. and I travel it weekly March to September... I have seen sub 20 mpg in thick snow...

One 'brim-to-brim' calculation is not accurate enough. There is too much experimental error in just two brimmings. I reckon you need at least 1000 miles to get a really accurate reading of your average - more if that includes unusually long motorway trips when most of the annual mileage is suburban. In May 2017 at around 120,000 miles on the clock, our Focus auto 1.6 estate used 113.27 litres to go 936 miles - 37.6mpg. But low mileage drivers shouldn't care about mpg - all cars of similar weight and power are much the same - other things are much more important.

Not sure what relevance this post has. I keep a running log of mpg and the variation in the values is fairly small. One or two 'brimmings' is fine if you want the mpg +/- 2.

The manufacturers mpg figures do suggest quite a large variation between cars in the same class.

I reckon you need at least 1000 miles to get a really accurate reading of your average

As we all know the engines of all cars "bed" in some sites suggest that the mpg improves as this process takes place, those sources quote up to 10,000 miles, some as much as 20,000 miles.

My experience is different to that in every car we have bought over the last 10. These are calculated figures corrected for any odometer error.

BMW 118d. After 1,000 miles the average mpg calculated as 48.25 mpg and 5 years 3 months later with 40,000 miles on the clock the average mpg was 48.29 mpg.

Seat Leon 1.4 TSi. After 1,000 miles the average mpg calculated as 45.15 mpg and 3 years 9 months later with 26,000 miles on the clock the average mpg was 45.26 mpg.

Nissan Note 1.2 DIG-S. After 1,000 miles the average mpg calculated as 48.15 mpg and 3 years later with 23,000 miles on the clock the average mpg was 48.08 mpg.

Skoda Superb 1.4 TSi 150 PS. After 1,000 miles the average mpg calculated as 44.13 mpg and 21 months later with only 10,000 miles on the clock (retired now) the average mpg is 44.21 mpg.

Skoda Fabia 110 TSi 110 PS. After 1,000 miles the average mpg calculated as 50.40 mpg and 7 months later with 4,000 miles on the clock the average mpg is 50.44 mpg.

So all very consistent and with the exception of the Note which drove far better as the mileage increased none of them drove any different as the mileage increased.

Then there was the Kia Ceed SW 1.6 CRDi 115 PS. After 1,000 miles the average mpg calculated as 47.33 mpg. After 10,000 miles the mpg had improved to 49.88 mpg, after 30,000 miles to 50.78 mpg and at 5 years old with 50,000 miles on the clock it had averaged 51.64 mpg. The wife used it for her commute (to the same place) for those 5 years and we used it for holidays and shopping etc, exactly the same use all the time. It drove far better the day we sold it than it did the day we bought it (it felt new and tight) and during those 5 years it had 1 ECU update at the 3 year service (30,000 miles).