How Accurate Is The Displayed Mpg?

[spawnhdi]

I am a new owner of a 2004 C5 HDI VTR saloon. Around town I'm getting a display mpg of 39.2, which I am very happy with. However motorway driving at 70mph I'm getting 39.5 average. 

 

There's no noticable lack of power, no power surges and no displayed faults. Are there any known issues which I can do to increase the mpg? Or is there anything I can do to the display to get a more accurate reading?

[paul.h]

Which size engine is your car ?

 

It is worth resetting the trip computer when trying different driving styles/roads so it is not affected too much by the previous trips since the last reset. This is done by holding in the button on the end of the wiper stalk. I find the mpg shown is not too far out compared to filling the fuel tank, driving and then refilling. Doing 65 mph instead of 70 mph will probably give another 10% mpg and probably about 50 mpg or more.

[spawnhdi]

It's the 110 bhp 2.0 engine. With the instant mpg showing, sat at 70 mph on a fairly flat road will show around 42 mpg. I've done a couple simple checks, such as tyre pressure and binding brakes. Could the egr be at fault? I'll plug my obd connecter in it tomorrow and see if there any logged faults.

[spawnhdi]

I disconnected the egr via the blue connector this morning to see if there was an difference and it's practically the same. Approx 42 mpg at 70mph. I now have a suspicion the egr make not be closing properly or is stuck. I think I'll either buy one of these egr cleaning kits or remove it and clean it properly.

[spawnhdi]

I also noted no change in power, but at idle when she was cold, was a little rougher.

[MikeLane]

Probably a stupid question but could the air filter be clogged? (Incidentally, when going fast downhill, if I take my foot off the accelerator the instantaneous mpg goes to 999 - i.e. fuel supply is cut off .)

[Randombloke]

To eliminate the EGR from the system, buy the £2.50 blanking plate and install it, if there is a lot more power or better fuel economy that's your answer.

 

My own view is that the Mk 1 sometimes had slightly binding brakes, worth having a look to see if the front wheels are free as the parking brake works on them.

 

A quick clean there can often be worth 2-3 mpg.

[spawnhdi]

Cheers random bloke. I've checked the brakes by feeling if the wheels had got hot, which they hadn't. I'm going to remove and clean the egr tomorrow. If there's improvement, I'll remove it all together at some point.

[spawnhdi]

Egr cleaned using wynns egr cleaner and the result....... exactly the same as before. Is the egr valve at the rear of the engine?

[spawnhdi]

So after a bit more research, I'm going to investigate all four wheels properly and then take a look at the maf.

[spawnhdi]

Maf unplugged in the way to work this morning. No power what so ever, but when I eventually got to 70mph the mpg was showing 43 ish. So next step is to give the maf a bit of a clean and see if there's any improvement. I've read some mixed reviews about cheaper replacement mafs, so if I do get one it's going to coat over £100.