Had a decent run in mine the other week for the first time in ages, 260 mile round trip to Silverstone and back. Most of it on cruise at 70 and mine averaged 22.3 MPG, that's pretty horrific even for a 3.5. No idea why it's that bad? Maybe just needs a good service? Going through the history a bit deeper and all the uk services have been done by dealers so i'm assuming that means oil and possibly oil filter only.