A 780 on a built 383 that is racing may be the correct carb, but this isn't a built 383, and he isn't racing. Is the 780 a double accelerator pump 780? A 780 is probably too big. Mopar only delivered a 383 with a 600 carb, and that was on a Magnum, this is a 2bbl motor.

Next, my 318 truck gets 13 mpg with 3:55 gears and 235 75 15 tires on an EFI motor being monitored by o2 sensors @ a very steady and smooth 70mph without OD. That mileage drops quickly when the speed exceeds 70. He has more cubic inches, more gear, shorter tires, and a big carb.

We have no idea how fast he was driving, or how he was driving, and we really don't know how much fuel he actually used. The whole deal could be based on incorrect info. A one time 67 mile test mpg based on guesstimated fuel consumption is pretty iffy.

At any rate, 5 mph with a 383 would suck unless it was a tire ripping monster. The tune is no where near where it should be with a carb that is plenty big for the motor. Checking to see how much fuel has been mixed with the oil is a good thought on anything that is getting poor mpg.