I drive a Cupra after upgrading from a TDI FR, I do about a 50 mile round trip to work 5 times a week which is all motorway driving. The part of the motorway which I use always seems to have a constant flow of 65ish mph, so I tend to follow the flow. At this speed I roughly get about 35mpg, best I got was just under 39mpg but this was proper granny driving. I fill up every Monday and it costs me about £50, but I normally have 1/4 of a tank left on the gauge.
The way I see it is I'm in no rush to get to work, so I will happily follow the flow. I have fun in the Cupra in the weekends.
Diesel torque
I'm personally of the opinion that petrols are pretty much unbeatable off the mark, but at motorway cruising you can't argue with the in gear power/torque of the diesel, 50 to 70 mph is absolutely effortless and you still return good MPG.
Not in the 1.6 tdi I had for the past week, at 50mph you needed to drop it down to 4th, only from 65 upwards did it start to pull in 5th. Mpg wise, driving it hard 56mpg, sensibly 63mpg that was over a 14 mile mixed traffic route. Back roads saw 68.1 mpg, over the 14 mile route a vw up 75pswas more efficient, 59mpg if you revved the nuts off and 66mpg driven carefully. I never got chance to give it a run. The up was actually quicker than the tdi, my cupra averages 29mpgover the 14 miles and I've never seen above 36mpg. I'd still prefer it to a diesel though.