Difficult to reply when you say it's fact but a good 2.8i returns what, 30 MPG on a run? Thats a mint motor IMO but a Rover V8 on carbs pulling a 5door bruiser of an SD1 returns the same if tip top some manuals a touch over
My comparison was when I junked the 390 Holley and Offy to go back to SUs as the cam and heads were stock and it was costing me too much dosh for town work. Cos the V8 has the good low to mid grunt in a light MK 1 Capri shell you could chug about and get 20 - 24 round town compared to the 2.8i having to be booted more to make the same progress, up and down the box and return a couple of MPG less. In saying that the Rover doesn't get much more return in the Capri on a cruise on the original 3 litre 4 speed as like you say they don't like to rev as much.
I've had a few 2.8s and the best I've ever got from 'em is 28 - 30 MPG
Anyways, the next motor I had in the following Capri (one I still have) was breathed on and set up for the 1/4 mile running a hot cam, 600 Holley with head work etc The best I had on a steady run from that lump was 12 MPG Hope to ressurect her soon with a better motor

@ DarylC (hope spelling right) I'll sort a thread with some old skool pics when I have sussed how to work it all here