An older motor like the '78 is gonna pull down some gas at a pretty good clip...not tryin' to be cruel, it's just a fact...I had an old Evinrude that would suck bugs and small birds into the fuel vents at WOT...so don't expect a whole lotta range w/the two saddle tanks...try to keep your RPMs down as much as possible and keep your load light and that's about the best you can do...one day a newer technology motor, but for now, just take 'er out and ENJOY!!...pics when you can...
You should see about 40 at WOT w/a light load and w/out a top...tops are great, but they are a real drag on fuel economy and speed...