I just got my 54 4100 up to 10mpg from 8mpg. That is on an 80 mile run averaging 40 to 45mph. Last big jump was getting the heat riser valve working properly. My carb is in good shape and I will run my timing at between 8 and 10 degrees before TDC. A new muffler helped a little, I think my old one might have been clogged a little??
The way I look at it, you spend money where it makes you happy. So I run the truck as much as possible - my wife is terminally ill and I need to escape every now and then.
Not sure what carb you have, but if it is a Rochester B make sure the power valve is working correctly, and proper gaskets have been used to pass the vacuum signal. Advance your timing as much as you can - I'll advance till I hear a ping then back it down a couple degrees at a time. 10BTDC seems to work well for my 235. I'll pull it back to 8 if I am carrying a load.
