What's The Average MPG for a 1984, 3.5 Ltr Petrol Range Rover?

Actually, I can tell you what it's supposed to do as I did part of my apprenticeship at a Range Rover dealership in the 80's and occassionally had to do road tests which measured economy.

A 3.5L Vogue EFI is supposed to do 24MPG, that's without aircon on.

Or shoes.

The OP did say "average", not "what's the best I could possibly hope for on a long journey though a deserted country at a constant 45mph?".

Realistically speaking, is he really going to get better than around 18mpg?

I'd say that was a reasonable expectation. Maybe even less round town/making progress.

Si