I put a manual OD trans in a '73 that previously was equipped with an automatic. On the highway, I was able to get around 18 mpg at best. Keep in mind, this measurement was taken with an inaccurate speedometer (could not find a speedo drive ration for the manual that would work with the 3.31 rear end) so the actual numbers might be a little off. I never checked the numbers for city driving, but 120 miles per tank seems about right from what I remember. The car was a real pain to drive in stop and go traffic, so I did not drive it much in town. Great highway car though.