are mpg computer accurate,im getting bad mpg

[codgod]

hi folks,there is a problem with my mpg on my 110 hdi.i know my tank holds 70 litres so on thursday i put 35 litres in the tank and that gave me exactly half a tank.ok so i reset my miles odemeter and the mpg computer and ive drove it around for 3 days urban/extra urban and the mpg counter says 42.2 mpg but ive only covered 89 miles but the guage has moved quite a bit (its almost on quarter of tank).so ive decided to do a brim to brim test.so ive just gone down the petrol station and filled it up it took 49 litres to fill it so that means ive used 14 litres (thats over 3 gallon)in 3 days to do 89 miles so how can the mpg be correct.ive just drove down motorway for 40 miles at 70-80 mph and the mpg computer says 53.3 mpg but the guage has moved a bit already i will do 200 miles in it then fill up again and i will work out correctly the cars mpg.what is wrong with my car ive not got a fuel leak,ive changed the oil,oil filter,air filter,fuel filter,it has new clutch new dmf,my brakes are not binding,tyres pressures are bang on,it does not smoke at all even if i floor it.the car is not sluggish at all it will fly if you want it to.i drive this car as sensible as i can 2000 rpm and thats it round town.so what could the problem be,any ideas would be helpful.thanks steve.

[rwb]

They are not, in general, accurate. Mine seems to be one of the worst; it under-reads by 8 to 12 mpg.

viewtopic.php?f=11&t=14772

[codgod]

but thats ok cause your mpg is good,in my case mine is over reading.

[rwb]

The only way you can work out the efficiency for certain is by brim-to-brim filling and working it out yourself. (Part filling won't do.)

When you've done this a few times you'll get a feel for how inaccurate the computer is.

[codgod]

YES I KNOW PART FILLING WONT DO.THATS WHY IVE FILLED IT UP.MY POINT IS WHATS THE PROBLEM WITH THE CAR TO GIVE ME POOR MPG,IVE HAD THIS CAR 3 YEARS AND I KNOW I GET POOR MPG I JUST WANT TO KNOW WHAT THE PROBLEM COULD BE.

[lozz]

What year is your car on mate?

[codgod]

hi lozz,my car is 2001 on a y plate 2.0hdi 110 exec estate.it has done 155,000 but drives superb it just seems heavy on the fuel.as i said earlier im doing a brim to brim test on it at the mo to find out what mpg its doing.i filled it up today until the fuel pumped clicked and drove down the m6 for 40 miles at 70-80 mph really smooth and plenty of power if i needed it and the computer said 53.3 ive done another 16 miles urban driving and now its down to 47.9.(i know that it will drop im just telling you the exact facts).the fuel guage now is just off the full mark and is now onto the white tick individual marks if you know what i mean.im going to run it for about 200 miles and then work out the calculations,what should i get for a full tank combined cycle cause these tanks are big they hold 70 litres! and its the first time in my life i have had a full tank! or what mpg should i be getting or what is the norm.cheers steve.

[lozz]

im getting 37mpg mixed driving out of a 02 reg hdi citreon its a much lighter car than a 406,
its had new clutch, remapped, new brakes etc, its crap compared to the old v reg 406 ihad on fuel,

im thinking these later model pug/ citreon hdi cars with all this low emissions dosers and all that crap are worse on mpg to those that dont have all that,

imight be wrong but thats what im thinking!

[Busman]

im getting 37mpg mixed driving out of a 02 reg hdi citreon its a much lighter car than a 406,
its had new clutch, remapped, new brakes etc, its crap compared to the old v reg 406 ihad on fuel,

im thinking these later model pug/ citreon hdi cars with all this low emissions dosers and all that crap are worse on mpg to those that dont have all that,

imight be wrong but thats what im thinking!

Having changed from a 99 HDI110 to a 52 HDI110, you could be right. I'm finding the mpg to be about 8-10 mpg worse on the newer car.

[lozz]

something else ive noticed is,

the diesel itself,

check and confirm foryourself to see if im right or not,

if you fuel up at for eg shell or another none supermarket fuelstation, you get more miles out of it, ( Supermarket, fuel dosent go as far,)
idont know what it is with there fuel but somethings not right,

[lozz]

ishouid get a jag,

http://www.jaguarforum.co.uk/viewtopic. ... 78&start=0

them lot are havin a moan, but look what Mpg there getting

[Bailes1992]

Getting a steady ~48mpg out of my 90.

People would say thats bad but the M4 on my commute is very hilly and a 90 has to be worked very hard indeed.

[piglet]

Can't get my 90 over 41, urban & rural . Computer usually says 36-37.
So 11% under read, and generally a bit poor in my opinion.

Is this true about supermarket diesel?

[gumby6371]

Can I just throw my 2 penneth into the mix,
Codgod, do not assume that because the book says you have a 70 litre tank that you do.
Back in the days before I needed a mortgage to fill up I used to brim the tank, run until the light came on and fill it back up. I regularly put over 70 litres in even filling up as soon as the fuel light started flickering at me.
In theory I had a couple of gallon left in my tank and squeezed another 70 litres in their as well.

The only sure fire MPG test is to brim the tank then run for a set distance and refill. The fuel gauge is a very rough guide on how much fuel is actually sitting in your 406.

I'd brim, run for 300 miles and refill 3 or 4 times to give an average, record mileage fuel etc and you should have a somewhere near accurate MPG figure after that.

Always remember the fuel gauge is basically the same thing that tells your toilet cistern when to pump some more water in after you've flushed it, accurate it is not

[codgod]

hi gumby,thanks for your reply,will do just that.will post with results.cheers.

Can I just throw my 2 penneth into the mix,
Codgod, do not assume that because the book says you have a 70 litre tank that you do.
Back in the days before I needed a mortgage to fill up I used to brim the tank, run until the light came on and fill it back up. I regularly put over 70 litres in even filling up as soon as the fuel light started flickering at me.
In theory I had a couple of gallon left in my tank and squeezed another 70 litres in their as well.

The only sure fire MPG test is to brim the tank then run for a set distance and refill. The fuel gauge is a very rough guide on how much fuel is actually sitting in your 406.

I'd brim, run for 300 miles and refill 3 or 4 times to give an average, record mileage fuel etc and you should have a somewhere near accurate MPG figure after that.

Always remember the fuel gauge is basically the same thing that tells your toilet cistern when to pump some more water in after you've flushed it, accurate it is not