Hi all. I have a 2007 3.6 TDv8 which I bought around 2 months ago. Since then I've been using it for my commute and putting around 70 litres of diesel a week in it.

The Dash computer reports around 26mpg, but a quick calculation shows I'm getting around 300 miles from 70 litres, which google informs me is more like 17mpg.

I'm doing mostly motorway driving, setting my cruise control to 70mph for the 20ish mile journey to / from work every day.

So......

Does that 17mpg seem in the region of 'something wrong' consumption?

If so, what could it be? Fuel leak? Turbo hoses?

Sounds like google has given you US gallons - they're smaller than UK so that's part of the problem.
70l is just over 15 uk gallons - making the actual result more like 20mpg (or just over).

That's still a bit below the 26 read out.

I've no idea mechanically but thinking logically - the read out on the dash is based on the distance the car thinks it's done divided by fuel flow.
The distance the car thinks it's done comes, I think, from the speedo/odometer which in turn comes from a sender off the gearbox. By law they cannot under-read, so any variable in that will be over so that could explain some additional difference from reality.

The other part is that the car could be using more fuel than it thinks it is (or should be) - I don't know where the flow is measure from but I would assume the same number is used by the ECU to control the fuel/air mixture. If that's the case then I guess air flow sensors could be involved to.

I guess fuel leak could also be plausible. 2012 RR Westminster TDV8
2008 Audi S5 4.2 V8
2006 Mazda MX-5 R/C Sport 2.0

Gone...
2007 RR Vogue SE 3.6 TDV8
2005 Ford Focus LX tdci 90
2006 Mazda RX-8
1996 Land Rover Discovery 300tdi
1998 BMW 523i
...

Ah yes, the us gallon thing makes sense, so I'm actually getting around 20mpg. Still seems low though for mostly motorway driving at a steady pace.

For the calculation of my actual mpg I'm getting distance from the odometer, so that should in theory remove one thing that could be different from the dash computer.

It seems like fuel flow could be the culprit. I'm going to check my turbo hoses as a first step, then brakes etc... try and rule out some obvious culprits.

First check would be tyre pressures and for any dragging brakes, also has the air filter been changed recently.

Lastly as the weather has been warm are you running with the AC on or worse the windows down, both will drop the mpg down.

Even in hilly Spain with AC running flat out I have never got bellow 28mpg and normally 30+ averages.

I ALWAYS get 30-32mpg on a motorway run of around 80mls in my 2008tdv8 3.6. 2008 L322 Vogue 3.6 TDV8 in Java Black with parchment and Navy interior. 125000mls.

I used to be indecisive and have a split personality... but we are ok now...I think?

Air filter (and all other filters, and engine oil) were all changed about a week ago.

Another symptom: vibration from front end when braking heavily, and there might (although it could be imagined) be a grinding feeling from the front of the car when driving.

Vibration could be worn/warped discs, are your pads worn right down? Sticking pistons in calipers, are the front wheels unusually hot after a run? 2008 L322 Vogue 3.6 TDV8 in Java Black with parchment and Navy interior. 125000mls.

I used to be indecisive and have a split personality... but we are ok now...I think?

Ah ha, Front wheels are extremely hot after a run. Threw some water on all 4 wheels this morning after driving to work, nothing from the backs but the water boiled off as soon as hit the fronts.

Is that new calipers then or can the pistons be fixed? Is it likely that both fronts would be sticking (both front wheels are hot) or is it likely to be something else if it's both of them?

Some cars have Brembo calipers which usually have 4cylinders and are usually reliable. If you have the "lesser" performance standard caliper it could be sticking. You can remove either quite easily too test and examine, refurb kits are available or you could source replacements off fleabay. If your pads are worn and discs are ridged, pitted, corroded, I'd replace em all.. Loads of info on here on how too.. Also consider wheel bearings, especially if you have noise that increases while negotiating bends at moderate speeds.  2008 L322 Vogue 3.6 TDV8 in Java Black with parchment and Navy interior. 125000mls.

I used to be indecisive and have a split personality... but we are ok now...I think?