The initial timing is approx. 8 BTDC but I don't know what the total is. ........ The next thing that we're going to check is to verify that the gas gauge is reading accurately. That would certainly mess up any mileage determinations.
If you haven't modified the slots for the mechanical advance I'd guess your total timing is too high... I was running a stock replacement electronic dizzy on my 318 and one day I realized that with the initial set around 10-12* BTDC my total was well over 40* BTDC!! If that's the case I'm sure you're losing a good deal of efficiency from the engine having to work against itself. Vacuum advance on top of that probably just makes it worse in this case...
Also just checking, how are you measuring gas mileage? Most accurate way I've found is to fill up the tank, drive for a while and note the miles, then re-fill the tank and note how many gallons it takes. This way you don't have to worry about an inaccurate fuel gauge...
