Before I did the big super duper MPG 318 build I had the bone stock 318 2bbl in my 68 cuda, it came with a 2.76 gear and cruised well below the 2400 RPM TQ peak it came from the factory with, now I had an otherwise identicle rear end with 2.94 gears and thought I would swap it in for a little more acceleration and to get it up higher RPM at cruise because like you I though it would get the best MPG cruising at the TQ peak, that is when I found out I was very wrong, cruising was right at 2400 RPM however MPG went down drastically, next rear end swap was to a 8.75 with 2.76 gears and MPG went right back up to where it was with the 2.76 7.25 rear even though RPM was once again well below the TQ peak. After that I built a 904 with LU and brought RPM down more and MPG went up another 1mpg.

After that I built a similar motor but with a few changes, the compression was increased, I added a 4bbl carb and headers, so the peak TQ RPM should have gone up significantly (I never dynoed it) and it definately felt like it did but guess what? MPG went up a bunch more. I got a best of 29mpg lugging the snot out of that motor well below the peak TQ. I really wish there had been higher gears available for the 8.75, I am sure I would have gotten over 30 MPG with that combo. That motor was not working hard at all to pull the car at that high of a gear, even at 70 mph (2200 RPM) it was still pulling 17 inches vaccume so there was way more power there than was nessacary.

Cruising at your peak TQ only gives you the best MPG if you have to run WOT to maintain your cruise speed at that RPM.