Previously posted in another forum, but so many errors and typos, I thought I would repost here and try for a response from the locals.
Here is my dilema. Quite simply, I seem to be getting lousy gas mileage. After a 300 mi round trip to So Maine yesterday, I determined I am only getting 10 mpg.
This eng is a newer rebuilt 4.9, (done in 03) with less than 10,000 miles. It is in an 89 F150, sb. 5spd, 4wd. (Rear axle ratio is 3.55) 31x10.5x15 tires.
I don't drive this hard at all. Avg 65 mgh, all highway, Am I expecting too much or should I be getting better mileage here? Is there an easy way to pin this down? Plugs, wires etc. Any advice will be appreciated.

 

How about the air filter and fuel filter. When were they replaced last?

 

Its been almost 5,000 since I changed the air filter. I have never changed the fuel filter. I plan to change both as well as invest in new plugs and wires next week. It's been running fine, other than the lousy mileage. I've also checked for leaks and don't smell any oder of gas, but not ruling that out either.

 

hey frank,matt h here.was reading ur thread and it made me remember back when i had a 1980 gmc shortbed.i turned that thing into an offroad monster.early on before any real work,the first thing i did was put on some used 31's and almost instantly my mileage went down the tubes.you wouldnt think so cuz their really not that big,but tis true.also my speedo went off by 7mph.if possible experiment with smaller stock size tires and see if it gets better,curious to hear if tires r problem.looked up ur gallery pics,nice truck!good luck matt h

 

I just arrainged to drop it off at the local garage in town. He is very honest and reliable. I have him checking the codes for a problem sensor. This weekend the truck will get new wires/plugs/air filter for starters. Need them anyway. Will let you know Tuesday how I fared. I know I can't go on getting 10 mpg with the price of gas around here. Hopefully it not just tires. I am guessing its the Oxy. sensor. See if I'm right...Frank

 

find a spot that has a measured Mile and see how long it takes ad 60mph to do it. If it takes you a minute then the tires are ok. if not then your off. If it takes you less then a minute then your going faster if more than your going slower. Or just find someone else and have then go a set speed and go the same and then that will tell you if your off or not

 

damn ur good!lol matt h

 

I been doin this forum stuff since 01 and I know a decent amount and been playing with my ranger alot more lately

 

To Matt and others who responded re low gas mileage. I am changing the 02 sensor. This based on putting the engine on a scope. Too old of a truck to have comp codes read. Result, needs new 02 sensor. Everything else checks out. I did put smaller tires on last week and have not driven it to determine if mileage affected. Will probably check that later this week. Hard to believe just changing the 02 sensor is enough to make a diff. but I'll let you guys know. Will be a while but will respond.

 

You do have a very good point there.