VW's are known to be better mpg wise but they run on £12 per litre engine oil so why wouldn't they be
that was my experience of them anyway,
As Cloth says never use the instant mpg it simply reports how fast it's going through it at any given time, my old 17 mpg Omega could report 999 mpg that way if the conditions were right,
What you say about the dealers is true as well, your car has to meet the exacting conditions but it's ok for them to sell you any old tat under a special name, keys are a real deal breaker and I won't buy unless there are 2 of them as the spare is now £500 to replace through official sources,
A fella who bought his where I got mine has had a rocker cover gasket leak and they've told him not to worry about it but keep an eye on it, what's the betting it becomes urgent the day after the warranty runs out or if he tries to px it,
Mine also imo doesn't meet what Volvo spec as Selekt, I've had to take it back for one issue already which they have sorted 99% and as the days wore on I noticed various scratches that had been hidden by colour magic or residue compound that weren't there when I signed for it, I've also got little confidence if I get a mechanical fault that they will actually fix it under warranty and I had a similar experience with a Skoda before,
One other thing I would suggest regarding the mpg is to feather drive so no hard pull always from standing and only pedal her to maintain momentum, I find mine is happy to drive normally using only the first 20% of the pedal, no real need to plant it at all, same thing on the motorway up to speed and then just gentle taps of the pedal if the speed drops off, adaptive cruise isn't economic either as you can feel the engine and brakes trying to overrule each other most of the time but it is a godsend in traffic that can't keep a constant speed, with normal cruise I used to find thx to bad ratio's that keeping to 70 meant not being in top gear in the Vx, it would hunt between 5-6 each time, slightest strain on the engine and down to 5th used to happen, automatic CDTi.