Hello: Done some calculations on my 66 Ford F100, I-6, 300, 1 brl carb engine, I came up with 9-10 MPG. Is that rate normal for engines like these, or something wrong there. I appreciate you sharing me the info. Thanks. Hamid.

I used to get 16 mpg on my 240 cu in 6 F100, but that was on real gasoline. It got bad like that once and the centrifugal advance was stuck in the distributor, check that with a timing light by running the engine speed up and looking for the marker to move ahead. Also split vacuum hoses can cause the vacuum advance to not work, make sure you have enough vacuum with a gauge, then find your leaks. I've had to buy a vacuum gauge from JC whitney, the auto supplies don't support them anymore, except the expensive hand pump model for transmission work.
If both those are right, some types of Reformulated Unleaded gas will run right through a carburator like water. The EPA prohibits the carb rebuilders from tuning the carburators to the type of gas they sell now, so buying a rebuilt carburator won't help. The jets are too big for RFG, they are right for the banned hexane heptane octane gasoline. I have never found smaller jets for 1 bbl carburators, or 2bbl FOMOCO carbs either. Besides, if you could find new jets, the old ones are stuck in the carburator so bad you'll never get them out. I've converted all my small block v8's to holley racing 450 cfm carburators, then changed the idle mixture and main jets smaller. I don't have a solution for 6 cylinder engines, I don't know of any carb adapter for engines that small. Old Gas tractors and lawn mowers have this identical problem, too much fuel used, black smoke, black fouled plugs, no solution available. Back in the 60's catalogs had 4bbl converters for 6 cylinder engines, but I haven't seen listings for this in years. Good luck.