Expect around 8 mpg, Hope for 11 mpg.
There's a number of things that can come into play... First...Make SURE the carb is tuned and set properly; No matter what you do, it's only going to accelerate so fast. Stomping the pedal down any further will do nothing but waste fuel. Don't worry about going 70-75mph on the freeways. You're driving a house. 60-65mph is really more than plenty, and will save a lot. Coast down the hills, and don't put your foot to floor going up them. If you lose speed on your way up, who cares, you'll gain it on the way down. You're on vacation the moment you get behind the wheel. Drive like you're on vacation time... which is when-ever we get there... we're there. lol.
Remember these we built when the speed limit was 55, and they're geared/designed to run best there.
