never believe those trip comps, especially the how many miles before you stop bit, as i found out trying to get a ferry in france with very little fuel. My mate (scary engineer type) worked out the maths, and proved the trip comp was way out. I will never let it go below 60 miles before u run out.
At 3000 rpm in 5th (70 ish), in my mk1 172, can get about 40 mpg going from yorks to london on a1 or m1.