Yesterday I finally got to drive my truck around just to check my mileage. I travel 58 miles round trip. My mileage came out to be 12.9 mpg. Half of the driving was at 50 mph and the rest was at 70 mph (just purring). My 1964 1500 truck has the stock 2 barrel carburetor, points ignition (engine is stock, I have not updated anything on it yet), 31.7" diameter tires, the SM420 transmission, and a 3.54:1 rear axle ratio. At 70 I calculate about 2650 rpm (I need to get my tachometer connected.).
