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17mpg is interestingly lower than mine- I get 23mpg with bikes on the roof and around 28 without, with an additional half litre displacement. Wonder if it’s a higher compression ratio or more modern engine management? Of course, you have two more cylinders- but smaller engine over all.
Two turbos probably don't help things. Or 4WD. I suspect it's significantly down to driving style though. I managed nearly 30mpg crawling home at a steady 55mph the other night.
At 17mpg? £30 wouldn't even get me to the end of the road. Full tank every time. Given the car weighs over 2 tons, there's no point in doing anything else.