Poor mpg when cruising?

[legoless]

Hello
A bit of a vague one this so here goes....
I am getting pretty poor mpg when trundling along the motorways just lately. My motor is a 2003 110 hdi estate (rapier) By poor I mean an indicated 37.4 on the trip comp when doing 70. If I cruise at 80 it gets a better 44 odd mpg. ( I have the air con off usually) I do have a magic fuel guage that drops quite quickly after a refuel but then gains a ¼ of a tank and then behaves normally. The engine does feel a little flat over 3000 so perhaps the 3rd piston in the fuel pump is going. I didnt see any metal in the filter housing though.
I serviced it in feb just before the mot was due which it just passed on the emissions. It had new air oil and fuel filters, new oil (total 2000 from stealers) ...all the usual service stuff . It drives very nicely. Suspension feels fine no wandering tyres are at 33 rear 36 front (michelins) with no uneven wear. No smoke visible on acceleration. No fault codes present. ITs done about 2000 miles since the service... I walk to work

Anyone able to give a check list to run through? I was thinking of getting it remapped at derv doctor but would a trip to the stealers be a better option if theres nothing to else to check
Any pointers greatfully received!
Cheers Al

[Gary406]

i wouldnt beleive what that trip computer says mate,

[legoless]

i dont but I am hitting the pumps more often

[Captain Jack]

Hm, it really does depend on where you're cruising. If it's hilly roads, keeping the foot down constantly up a hill to maintain speed uses quite a bit of fuel. This is especially true on a cruise control rather than if you're using your foot.

I've driven some long distances last week and my average (on MFD) was flipping betweek 47.9 and 48.7 MPG. This is mixed motorway (between 70 and 90mph) and town driving. I'd check the obvious first, like fuel filters and oil levels. Replace it if possible. Also, check your brakes. It could be that they are binding to the discs and causing the engine trying to move the car on brakes..

[highlander]

Make sure your tyres are at the recommended pressure - should be 34 PSI on a 406 saloon; not sure if it's the same on the estate. You'd be surprised at the difference it can make.

[eoin27]

Thats pretty shocking MPG. i could remember getting about 57mpg+ at 70mph from my HDi. I easily get 45mpg from the coupe at that speed. I have a light foot though and at 60mph it never falls under 50mpg unless a hill presents itself

[Busman]

I get 57 - 60 mpg with the aircon on. The aircon makes mabey 2 mpg difference at the most on mine. I would check the accuracy of the trip computer with a tank full to tank full measurement as a start.

[sirwiggum]

Getting about 37mpg on my HDi 90.
But then, it is turning out to be a bit of a lemon.
Not as bad as the 20mpg of my last car.

[legoless]

thanks for the various tips.. the only one I havent had a look over is the rear brakes so something for the weekend...

[Welly]

I serviced it in feb ........ Its done about 2000 miles since the service... I walk to work

I would say THAT is the reason for lower MPG, lack of use.

Do you reset the mpg-o-meter when on the motorway to get an average for that trip only? if you leave the computer to try and catch up it takes for evAr......

[legoless]

Yup Welly
I ve noticed that I can make my usaul periods before having to fuel up... I always brim it. driving style hasnt changed .. if anything I have been trying to drive it light on the pedal just see what I can eek out. I havent checked the brakes .. doing that later today. Is there a way of checking the injectors in case one of those isnt working correctly? I will chuck some forte at it too for help.