About 7 years ago I inherited my fathers 1987 F150. It has a 4.9 engine Auto Trans. It is a really nice truck with about 75000 miles on now. It was running pretty ruff and I did a good tune up on it and replaced some vacumm lines but noticed it was only getting about 12 mpg also when crusing at 60 mph according to speedometer it was really going about 51mph. I put as high of a side wall tire in 15 inch that I could find and keep it stock now at 60 I am going 55. I am starting to realize this truck was like this from factory. I am also thinking I could be getting 15 to 17 mpg out of it. I have been doing a lot of reading and finding there are a lot of sensors from the engine to the exhaust that could be bad along with a cat converter going bad (orginal exhaust system). This engine is fuel injected and was also told that the injectors could be going bad. Really want to keep the truck but, can not afford the 12 mpg out of it and the dealers only want to sell new one's . Can not find any body with serious answers on some of the problems I am having. One is terrible fast idle, engine sounds like it is screeming at 65mph, seems to have power but still stalls out at times and I think the engine should idle smoother than what it does. Lot of small issues which may add up to a lot when done but, this truck is a peach, really nice to drive and solid. Do I need to start replacing all of these sensors and injectors.

 

Did you calculate the speedometer error when you calculated your fuel mileage? 9 mph at 60 is 15% and that will reflect on your mileage calculation. Odometers tend to be accurate to the indicated speedometer reading.

Is your check engine light on? have you read codes? Don't throw parts at it!

I had a 92 Bronco with a 4.9 years ago. It managed around 15 mpg.

Is your transmission shifting correctly? How fast is the engine turning at highway speeds?

 

It sounds like you have a vacuum leak. That would give you rough idle, occasional stalling, and your gas mileage would go down too. Have you looked over all the plastic and rubber vacuum lines? They're probably in need of being replaced given the truck's age.

 

I have always gotten a Haynes manual for any vehicle I ever had. A lot of useful stuff in them, although they are not the best around, they are handy. My 92 was doing something similar to yours as far as mpg's. Got a can of starting fluid a gave it a squirt here and there to find my leak.Engine will rev up a bit when it injests the ether.Found out my upper intake had a leak. Also the manual will tell you how to get the codes and what they mean. They are only a starting point and don't necessarily indicate the exact problem but they give you a good idea whats up. Good luck.