I purchased a 1996 4.9L 6 cylinder 5 speed manual F-150 about 6 months ago. When i first purchased it i figured that it would make it look cooler with 32 in. tires instead of the 26's that where on it. BAD DECISION it now has about 110,000 miles on it and it runs at about 10 miles to the gallon in the city and almost worse on the highway. Since then i have replaced the air filter, gas filter, egr valve and sensor, plugs, wires, and the distributer cap and button. It runs fine but i wondered if anyone had any ideas on why the gas mileage is so bad? If i can't fix it soon i think i'm gonna sell it and get a different truck.

 

what are your rear axle gears? theres a lot of extra weight with that extra 6 inches of tire that that inline six has to push, so gearing will be kinda cruicial.

If you still have them sitting around, forego the "cool" factor and put the original size tires back on, see if it makes a difference.

 

You doing pretty good...I'm pretty much stock with the tires and all...I get about 8 MPG....and my foot is not that heavy. Any ideas?

 

any codes? does it smell like its running rich?

 

No codes, no smell....just sucks gas like a vacuum

 

I wouldn't consider 10mpg on the 4.9L very good at all. I get 13-15 with my 5.8L with 33 inch tires and 3.55 LS gears.

zmadelem, Nighthawks mention of rear end gearing should not be ignored... What are you packin in the rear pumpkin? Gearing will make a huge difference in mileage. Basically, bigger numbers = more power and worse gas mileage.

That being said you still should not be getting 10 mpg on a six cylinder no matter what gears you have in my opinion. You haven't noticed any problems with the way that the truck runs??

-Chris

 

My fiancee's 92 F150 with a 300 I-6 and manual trans. gets about 15mpg city, 22mpg highway. My 92 F150 with a 302 and manual trans. gets about 10 mpg city, 14mpg highway. Both trucks are bone stock. Her truck has 140k miles and mine has 78k miles.

 

I thought 10 was really bad too, I was thinking some wierd combination of 5.13 gears or something for some odd reason. I get the same with mine....87 F150 w/1994 5.8 conversion w/ZF conversion & 3.55LS and 32s

 

First of all if you have some tall gears like a 3.08. Its probably straining the motor all the time so thats one reason youre getting bad mileaga. Second, since you added the larger tires you odometer readings are probably off. You will have gone farther than what it says, giving you a low mpg calculation. I hardly think you should sell the truck because you put the wrong tires on it. Either put the origional size tire back on or install lower gears to handle the larger tires.

 

I completely forgot about the speedometer thing, thanks for throwing that in. Find out how far you're speedo is off. I know with mine, its around ten off.

 

Being at risk of sounding very stupid, i just wanted to know how to find out what type of a real axle i have. I am 17 and this is my first car, so i'm new at this whole repair thing. Also i will check to see if my speedometer to see how far off it is, any other suggestions?

 

Actually before about a week ago it was running very bad. I got some codes that said that i was running on all my cylinders, it couldn't maintain 65 on the high, and i had emmsion issues, but since i replace the plugs, wires and distributer its been running fine other than the gas mileage.

 

Don't sweat it man, only 19 here, but because of these guys here and the fact that I'm always working on mine (whether it needs it or not) you can learn a whole bunch in not that much time.

More than likely you have the 8.8" rear axle. There should be a metal tag attached to one of the bolts on the differential cover, get the numbers off that and let us know what they are.

You said you got some codes.....Which codes did you get? When you replaced everything, did you reset the computer? The problem might have triggered "limp home mode." Disconnect the neg battery cable for about 30 minutes or so, take it for about a 10 mile drive of regular driving, bring it back and re-pull codes. Let us know what pops up

 

actually after i replaced all of the parts i did reset the codes, and nothing has come up since then so i'm fine with that, uh tomorrow morning ill check the tags and tell you what type of axle i have, its like midnite right now tho

 

Z-mad,

There will or should be a tag on your rear end, uh, I mean your trucks rear end with a spatterin of numbers on it that will translate in to your axle ratios. When you find those, you may be able to translate it out of a repair manual or by getting back on this site and asking someone to help you figure it out. Pretty easy stuff. You'll find that the folks on this site are good eggs and very helpful, even with "stupid questions" of which there are really none. Only those that aren't asked. Trust me, I have asked a bunch. Now to check your speedo, the next time you see a cop, speed up to 100mph indicated. When he stops you, ask him what he clocked you at and then do the math from there to determine how far off your speedo is. I say 100mph because it is easier to do the math in multiples of 100. Basic division that you probably got in school. Okay, I am just kidding. I usually check my speedo when I pass those "Your Speed Is..." signs that the cops or highway department puts out ever so often. They should be fairly accurate and might give you an idea of how far off you may be. Welcome to the site, and get ready to learn!

Rich