Wow. Some interesting maths on mpg

[golfmk4owner]

Well frankly that's how a diesel should be!

Anyone got any thoughts then?

[Agreeable Slick]

There are numerous reasons why it will be out by a certain percentage, especially in the colder months.

There are all kinds of things to do with the fuel itself, waxing points, specific density at differing temperatures, efficiency of the fuel filter, fuel pump etc.

The other thing of course is that these MPG readouts are more of a convenience for us as drivers, and the ECU is only measuring the things it has been told to. It gathers information from things like pulse width of injection, fuel flow FROM tank (remember that a lot of fuel in summer months gets sent back to tank), engine load, and road speed. What is worth mentioning is that on the live display as soon as the throttle is released the live MPG goes out of range as the engine does not pump fuel as it uses inertia from the wheels/flywheel to maintain the spinning of the engine. This is logged by the ECU which will artificially inflate your MPG figure of course. I found that going down certain roads around me I could ramp my live MPG average to 100MPG+ due to going down hills with no throttle.

I think it is safe to say that as winter approaches you can utterly disregard the readout from the computer as the log points are not representative of what you drive, I.E. it could miss a short trip to the shops, but would update numerous times on a long motorway drive.

There is of course the issue of active regen if you have a DPF.

HTH's. 

[golfmk4owner]

That's an interesting point. So basically, from what i've learned and experienced myself, my conclusion is that the mpg computer may as well not exist. I would expect a degree of error, but if its there "as a reference" but the reference is nowhere near then it's a waste of time. Think im gonna clean the maf, change temp sensor and give the egr a clean too, then see if that gives me near 500 miles to tank. If not, i might jump on the diesel hating bandwagon.

[Agreeable Slick]

It's worth having as it gives you a ball park figure. Including percentage tolerance/error which increases in winter.

However, I would look more carefully at yours before going mad about the MPG computer, as only managing ~400miles to a full tank is pish. My 130 Passat estate does a minimum of 500 to a tank and is only 55 litres like yours but has a bit more bulk to carry around. I do drive like a grandad in the mornings though as I'm too sleepy to be bothered with driving avec spirit.

Thats mostly ~10miles each way to work, no motorways or A roads either. A couple of reasonable down hills though.

[golfmk4owner]

Yes i agree, but 54 compared to 38 is not a ball park figure... it is an optimistic near paper figure plucked out of thin air, therefore useless.
Defo going to investigate and if things don't improve, the car is gone.

[Agreeable Slick]

I will assume that the MPG read out you were going from was from position 1 on the switch and not position 2. 1 is that trips MPG and 2 is an average over extended time (unless you cleared that as well when resetting the MFD)

[golfmk4owner]

It was off number 2, which i reset when i filled my tank so i could see what i had over that tank full of fuel. So yeh it definately told me 54 for that tankfull!

[Rhyso]

Dont clean the MAF - doing so generally shortens its life span considerably. Clean the EGR valve, get the tracking checked and make sure your tyres have a decent amount of tread and the correct tyre pressures

However with winter setting in expect your MPG to drop unless you fit smaller, lighter wheels like I have done and continue to enjoy the same level of MPG 

[Garza]

I easily get 600+ miles per tank in my GT TDI 140. 200 miles of that a week is motorways, then weekend miles which is variable driving.

[golfmk4owner]

continue to enjoy the same level of MPG 

I'm not enjoying my MPG.... its terrible for a diesel. That's my point.