My 250 gets it's best mileage around 2400 rpm, that gets me to 65 mph with a 3.08 gear and 28 tires, no lock-up or overdrive. Vacuum stays above 10 if on level road, and little wind. A head wind will knock off 1 to 2 mpg. Last trip out it got 20.5 mpg, truck weighs 3450lbs full of fuel and two on board.
Best thing for fuel mileage is an Air Fuel ratio gauge, I picked up 6 to 7 mph by tuning the W-1 carbs, mostly leaning them out at cruise rpm. On level road at 65mph, it runs about 15.2 - 16.0:1 a/f ratio depending on temp, wind, and which fuel station I was at.
With a A/F gauge, you will know right away if its geared right and jetted right.
Something else I did was make a cardboard throttle position gauge taped to the dash. I had a moveable needle with a spring on it pulling back to zero, the other end had a string tied to the throttle pedal under the dash. I marked idle, and full throttle, split the rest evenly with a few dashes. I found 95% of my driving used the first 20% of the throttle, this then told me where I needed to change the metering rods. W-1 Carters use mechanical means to richen the mixture with three step metering rods. Since I have dual Carters, a .030" change in height of the two metering rods, will change the A/F ratio about 1 point. It takes a dial indicator mounted on a plate on each carburetor to set them accurately. A 1/32 of a turn of the idle screws changes the idle a/f ratio 1 point, sometimes it hard to tell if I turn them, I have to look at the gauge to know.
Having the idle and run mixtures at or above 14.7:1 ratio will increase your mileage more than you think. Mine ran good and was getting 14 mpg on the road, once leaned out and adjust right, it jumped to over 20, and that's at 65-70 mph!
