I could get 40mpg with my V8 SD1 Vitesse, once in 5th gear you could drive all day without changing (almost)
I once did the journey from home, just outside Ayr to Castle Douglas a distance of approx 70 miles. I pulled away in 2nd gear then up to 5th when leaving home and the next change was back to 2nd gear in Castle Douglas.
What a car, a genuine Rover V8