I just bought a 1990 F150 with an 8ft bed for my business, and I noticed that it seems to be using a LOT of gas. I've found no leaks, and the truck seems to run great, but it's only getting about 10-12mpg! This is combo driving on the Highway and City, mostly Highway, though. It has a 300c.i. fuel injected 6 cyl, with the T-18 4 speed. Unless I have it loaded, I don't use 1st starting out. Is this normal mileage for this engine?