I filled the car up last night and though I'd work out my real world MPG. Was reassuring to see my trip computer showing an average MPG of 58.2, but when I worked out the real world MPG, 46.3! I know that the trip computers always overestimate the MPG but by 12mpg?! That seems a massive gap. My numbers are below: -

41.42 litres used in 422 miles.

Also disappointed at the real world MPG of 46.3, that included mostly 60mph driving on country roads and a long motorway drive. Changed all the filters in the car last month too so should be running OK. Put some Forte Diesel Treatment and Turbo Cleaner in at the last fill-up and usually put in a shot of Millers Diesel Ecomax at every fill-up (not recently though). Always check tyre pressures too and they're spot on. I am getting a rattling/whirring noise at 1500-2000 rpm though. The mechanic who did my MOT looked at it and said that it was the turbo but nothing to worry about. Don't know whether that's affecting anything. Anyone have any ideas?

If the trip's in mode 1, it effectively gives you "current journey mpg" (all segments since you last stopped the engine for at least 2 hours). If you were just off a long cruise or relatively gentle trip, that will be part of it (I've had my Octy for 7 years and know this can give me 10mpg better than my usual commute), and the trip mpg figures are usually ~10% better than the actuals.

I always have my trip computer on mode 2, so it averages since it was last reset (I reset it at every fill-up).

It's common for the trip computer to be inaccurate. But they don't always over read. In my Fabia the trip computer under reads by 6.5%.

My long term average in my PD100 is 62 mpg, but I drive it carefully and only really use it to commute to work on A roads/Motorway very early in the morning, so I never sit in traffic. Having said that, I slept in a few weeks ago and had to drive to work like I stole it.....complete with very high speed cruise on the motorway. I got 47 mpg (corrected trip computer figure) on that journey!

Your figures don't seem beyond the realms of possibility, but to be certain you need to carry out your experiment another 2 or 3 times to reduce error from slight variations in brimming figures. 46.3 mpg for the type of driving you do doesn't sound great, but I'd confirm it with more brimmings before investigating it.

I went from junction 6 of the M25 to Doncaster. The tank was brimmed prior to leaving. I filled up again when I got there. The trip readout was between 49-53mpg. When I worked it out it was 52. I did the same on the way home, it read 53mpg nearly all the way home. When I did the calculation it was pretty much spot on 53. This was all at between 70mph to +15 on that. It was good enough for me. To be honest, there have been other times though when I have been doing cross country driving and its said 58-60mpg. I know that deep down, its lying to me.

Mine under-reads, too. Has done for the last 9000 miles consistently. Some just vary!

I drive with the 'live MPG' showing as you will be amazed how much a 1mph change in foot position impacts MPG. There are some weird anomalies where the engine seems really inefficient and are worth avoiding.

Of course, its very much down to the individual and how they drive as well. Which I know is stating the obvious but when I have been in cars with other people and see how they either labour engines or hold them in a lower gear for too long it will make a difference.

Thanks for the replies. I'll just have to keep an eye on it I guess. I've done the elephant mod but that hasn't made much difference I don't think. Just concerned about the MPG as I'm driving to the south of France in August so want my MPG to be pretty good for then. Hoping there's nothing wrong with MAF, EGR, etc.

Also, WIND! Has a huge impact. I was doing a show a month or so back and driving the same stretch of the A1 from north london to Peterborough twice a day. On the way to Peterborough, northbound, I could easily get high 50s and that would be at rush hour. On the way back I would struggle for mid-50s with better flowing traffic (and speeds of 60-70) which I put down to the drop in temperature and that the winds normally blow northerly!

Also, WIND! Has a huge impact. I was doing a show a month or so back and driving the same stretch of the A1 from north london to Peterborough twice a day. On the way to Peterborough, northbound, I could easily get high 50s and that would be at rush hour. On the way back I would struggle for mid-50s with better flowing traffic (and speeds of 60-70) which I put down to the drop in temperature and that the winds normally blow northerly!

That's a good point actually. It's been very windy on my commute in the past few weeks, didn't initially think it could be a factor but it makes sense.