It may very well be just my imagination, but 3 months ago i decided i can do 30 miles per hour on 3rd gear and not shift to 4th. It didn't seam like the engine was under any extra stress or anything, it was doing 2400 rpm (an around 1850 on 4th). I'm driving a 2005 C4 so my on-board PC tells me how many miles more I can do with the fuel in the tank For the month I only ever used 3rd for 30 mph I did 350 miles, but every other month before, and the two after (that I was going all the way up to 4th gear) I've been getting solid 400 miles per full tank.
Hence the question from the title: Is there correlation between fuel efficiency and preferred gear at 30 miles per hour.
Some extra info: I live in the UK, and always, always, always have my speed limiter ON.