Yes, plus the relentless hills you've had to drive over once you're past the Somerset flats; Exmoor, Dartmoor and then Bodmin moor don't help with economy (assuming you went M5 and A30 not that it makes that much difference).
Where else are you visiting on your trip?
Actually, economy has been reasonable, 38+mpg, pretty good in the circumstances. Been to Lands End today, now in Newlyn. Great time had by all in the Red Lion with the pool table and juke box. Breeders Cannonball, hadn't heard that for a while
I'd call 38mpg pretty darned good
My dad lives in Newlyn funnily enough.
Pop over to Mousehole if you get time, it's beautiful, with a lovely flat walk in if you park on the road on the way in (free parking) allowing you to stroll along with lovely views of Mounts Bay.
Marazion a must do too, park in Long Rock and walk in ideally but there's a big car park with views of the Mount just on the way in. Pick up a Philps pasty if you do go there, or if not pick up a Philps from Hayle on the way back out of the area.
There's just so many great places to visit in West Cornwall.
Oh, if you're going to St Ives I'd suggest jumping on the train at St Erth rather than driving in. It's very cheap and if you sit on the right hand side of the train on a nice day you get spectacular views of the Hayle Towans and St Ives across the water as you head in.