(T) Accuracy of MPG readout

I have a X reg 105 ELX diesel.
37.85mpg

Sad I know but I have kept all my fuel receipts from when I bought the car back in December. This figure takes in having kayaks on the roof, bike rack, motorway driving, driving like a maniac etc (not all at the same time by the way!!)

8648 miles, £1000.24 on fuel, 1038.64 litres of fuel, 228.468 gallons

I'm getting a reading of around 40mpg with a mixture of motorway and city driving, dropping to around 36mpg on urban only. I'm not particularly lead-footed and stick to around 60mph on the motorway.

On a recent drive from Liverpool to Hereford (90% motorway got an overall reading of 48mpg).

I think it is an average rather than an on the fly reading. The longer you leave it without resetting, the less it fluctuates, which would tend to support the notion that it is and average.

I'm getting an average of 41mpg through 25 tanks of diesel. This is brim to brim. Don't even bother looking at the computer anymore.

Interestingly, I just had the alloys resprayed under warranty, and drove it for a week after it was returned without checking the tyre pressures (it was also very hot and I used the aircon a lot!). When I next filled up I checked the tyre pressures and they were all nearly 1 bar down in pressure.

On checking my mpg on this tankful it was 37 mpg, the lowest ever and something of an anomaly. Curious how much difference low tyre pressures (along with aircon usage?) can make.

Needless to say that the mpg looks much better on this tankful.

It does appear that nosejob Multi's have a smaller fuel tank than uglybugs. I used to get 600 miles from the uglybug but struggle for 500 in the nosejob but get similar mpg's. And (obviously) I get noticeably less fuel into the tank, from virtually empty, in the nosejob.

Hi. It's really interesting reading the different MPGs everyone has been getting. Not done a brim to brim on my Multi (02 1.9 JTD 115ELX) but have been checking the computer readings for some time now.

Had the car for about 4 years now and experienced all the routine multi problems. Leaking sun roofs, broken spare tyre winch, faulty brake disc sensor, rear wiper failure, 2 safety recalls - front springs and handbrake cable fouling, faulty clockspring, indicator wont turn off........ then to top it all a snapped cam belt (my fault as missed the recommended replacement by about 10k miles - although should have done it at 50k really). Anyways - still love the car.

Sorry - getting back to MPG. Before the cam belt snapped I was getting about 36 mpg on the computer - this involved mostly mixed runs of about town / country and motorway driving.

After the cam belt snapped I had the car repaired at a local garage rather than a main dealer as I didn't want to take out another mortgage. Agreed the price up front after the head had been lifted to look at the damage - £500 all in which I thought was a fair price considering a number of the valves obviously needed replacing and all the other parts - belt kit / tensioner / hyd lifters. When I got the car back it was running at about 33 mpg on the computer. I figured that the damage to the engine had obviously caused the poorer fuel consumption figures.

A month or so ago I put the car in to the same local garage for a full service - new oil / air / fuel filters etc. The car is now doing between 39 and 44 mpg which is the best it's ever done so can't really complain. Reading this post it may actually be doing better if the computer is about 10% under.

Interested to know what other users have found to be the best fuel / tyre pressure etc to get the best mpg - I've tried total garage excellium diesel which claims to give you 30 miles more per tank but can't say I've really noticed the difference to be honest?

Our 06 JTD seems to do 40-43mpgWhich is a almost all b road and a little town city driving.I am happy enough with thatOf course with the wife blasting out the climate control in this heat it won't be so good this tank full

Just returned from a weekend in Warwick driving 5 up on the motorway at around 70mph - returned 46mpg according to the computer. Happy with that...

those with diesel who have poor mpg, might be worth disconnecting the MAF sensor temporarily to see if the Mpg improves. It seems like there are a lot of overfuelling MAFs out there.
If MAF working well, mpg should be slightly worse if disconnected.
If overfuelling, it will improve - indicating it needs changing.

I'm not sure of the type of MAF on the JTD, but on other cars the thin wire can be carefully cleaned with spray alcohol. But people like Roverron (see tuning diesels.com) advocate that mafs may be effectively a service item with a 50k ish life.

and another thing!
after reading this through again, there MUST be some way the dash computer can be accessed manually to apply a correction factor. This was a common procedure with the Rover 75/ZT after people found out the buttons one needed to push to get the dash readout!

Someone somewhere must have access to the procedures for doing this in the workshop?

05 plate 83k JTD
first brimful test - 51.2mpg; computer reading 39.7
(previous reading had been 37.5 over 1700 miles, so say 48mpg-ish actual)

well - seems to bear no relation to reality!
second fill - 40.6 on dial - 46.85 actual
third fill -42.5 on dial - 42.3 actual

and latest: 37.6 on computer - 48.37 actual ! I wish I could stop pressing the computer buttons, but its too addictive - like have a joke a day on the calendar.

I was thinking about this the other day on a particularly boring trip up the M40 (Beaconsfield to Leamington Spa).
I was checking my odometer reading against the mile markers.
It was clocking up a mile (on the odometer reading) against approx 1.3-1.4 miles travelled (according to the mile markers), so seriously under-reading the mileage done!
I didn't have anyone with me unfortunately, otherwise I would have slowed to 60 (indicated) and timed the period between the markers to check the speedo reading.

So, if the computer is 'seeing' less miles than it's actually doing, then the MPG calculation will be low.

err... so does that mean we are servicing our cars too regularly and selling them too early too?! (i.e. does the total mileage shown link to what you have seen - presumably yes!)

Actually it's the other way round .
It's doing more mileage than it's showing on the odometer (in my case anyway.
E.g. Mine is only showing 1 mile for every 1.3 or so actually travelled.

That is if we can assume the Motorway markers are accurate of course, I'd think they should/would be.
I'm going to check it again when I get the chance, on another stretch of a different Motorway.

Guys I can tell you the marker posts your talking about on the motorway are actually every kilometre and the orange sos phones are every mile.