I have a 318 with 80k miles and the gas mileage is atrocious. In small town driving about 5 mpg sometimes worse. Hwy 15 mpg.
I know this is not a modern fuel efficient vehicle BUT I feel like it should do better. The truck runs good and idles pretty smooth. I put a new dizzy cap and plugs in and other than maybe running and idling a little smoother there was no difference in mpg. It does seem like it's running a little rich.
Is this normal? If not what should I be looking at to improve mpg?

 

Get up to about 30 mph (50-60 kph) and slip it into neutral. Does it coast fairly well or slow down quick? It could be something in the brakes is dragging. If the brakes aren't dragging, I'd first make sure the air filter isn't stopped up. 5 mpg is pretty low for your truck. I had an '84 and it got 10-13 in town.

 

Also check all along your fuel lines for a leak.

 

I have seen issues with this before. Either a cracked rubber hose or pin holes rusted in the metal lines. It will run okay but under acceleration, it will run out of gas, literally. The little holes will allow air into the fuel lines. Normally this won't bother the engine but under acceleration it will let it run lean. This ill cause a loss of power, but probably not lack of economy.