I just recently purchased a 94 f-150 2WD with a 4.9L 300 I6, std 5spd, I think it has 3.08 gears cause the axel code is 18 but the truck has had six prior owners one of which was a contruction company, so I'm not sure if the gears have ever been swaped out.
Just calculated my milage and it is 12.5MPG, does this sound right to you? The truck is EFI and mostly city driven and not driven hard.
This seems very poor to me for having 3.08 gears.

 

No, from what I know you should be getting about 17mpg, unless you have a rear end that is 4.10 or higher. Somebody correct me if I'm wrong.
Brett

 

Im running a 4.9 with 4spd and i have 4.10 rear end and roughly get that milage. i do a bit more highway driving. Look at the tag on the actual rear end. it should be attached by one of the bolts. But beware they might have changed gears and left the old tag on.

 

Do you have a tachometer in it? Pick a speed in 4th gear and give us the RPMs (I'm assuming that with a 5 speed, 4th gear is the 1:1. I have a 4 speed so I don't know for certain.) With that and your tire size you can calculate your rear end. If you DO end up having a 3.08 rear, then no, ~12 mpg isn't normal.

With my '81 carbed Bronco, 4speed, 3.00 rear end, I can get around 15 - 16 in town.


Let's say you're going 50 mph, look down and your RPMs are 1600, your tire size is 32", then you'd just punch it into the following formula:

Final Drive = (RPM x TIRE DIAMETER) / (MPH x 336)
Final Drive = (1600 x 32") / (50 x 336)
FD = 51200 / 16800
FD = 3.05

Basically would mean you have a 3.08 FD.

Now if you look down, you're going 50, your tire size is 32", and your tach says 2100 - 2200, then you have a 4.11. Of course, keep in mind this is in your 1:1 gear, which I'm assuming is 4th.

 

12.5MPG is a little low for that combo. I can get around 12MPG towing a trailer with my 80 F-150 300 C6 and 2.75 rear end. 15-18MPG without the trailer. My final drive should be near the same as yours, assuming it has 3.08 rear and O/D.
I'd say start with checking the basic tune up items, and insure that the exhaust system isn't restricted. After checking those, look for brakes dragging or under inflated tires.
Also have a look at the fan clutch. If it has locked up, it will cause a severe hit on fuel economy. The engine not getting up to proper operating temp will also adversely affect MPG.
These are just a few areas to look for the cause, and I'm sure there numerous other items that can be causing this issue.

 

are you going by what the manual says the gas tank size is and watchin the needle?
Or are you filling up the tank, driving it a certain distance and then filling up the gas tank to see how many gallons you used? Because thats the most accurate way. Plus the tanks arent as big as it says they are in the manual.

 

Squeezed 33.5 gallons into mine. =P

Also keep in mind that it's possible that if they changed the gearing, your mpg isn't going to read correctly either unless they recalibrated the speedo. For instance, if your odometer says you traveled 350 miles on a tank with 3.08 gears and then you switch to 4.11 and don't redo your speedo, the odometer is going to say you travelled 260 miles. If you used 20 gallons of gas, 350 / 20 = 17.5 mpg, whereas 260 / 20 = 13 mpg. You still went 350 miles on 20 gallons and got 17.5 mpg, but with the different gears it's going to READ that you got 13 mpg.

I'd calculate and find out what your rear end ratio is before you do any more mpg figuring.

 

Changing the gear ratio in a 94 won't affect the speedometer. Only a tire change will affect it. I agree though, we need to make sure that you have the gears you think you have and you are getting the mileage you think you are.

 

Check your speedometer and odometer on the freeway using the mile markers. Hold a steady 60 mph and you should be doing a mile per minute on the speed and one mile between mile markers. Best done over a several mile stretch...ten miles would be better.

 

I'm getting roughly eleven, and mine's stock 3.55, snow in the back, 31" tires. My fuel pump is going out, will that hurt my milage?

 

Ok my tires are 235-75r-15 which are 28.9 inches when you convert them to inches.
At 50 mph in 4th gear my rpm's are at 2100, so using AbandonedBronco's formula,

Final Drive =(RPM X TIRE DIAMETER) / (MPH X 336)
Final Drive =(2100 X 28.9) / (50 X 336)
FD= 60690/16800
FD=3.6125

So does this mean I have 3.55's or 3.73 or something else. I've been calculating my milage by filling the tank, then driving it till it's empty then refilling it and recording my milage along with how many gallons it takes to refill over a six week period. keep in mind I live in Canada so the temp is - 20c or -7F for you Yanks lol, so I usually let my truck warm up for 10 or 15 mins before i go anywheres. So I'm sure my milage will be alot better in the summer, but I'll have to wait a little while before I can find out.

 

I'm going to take a pretty sure bet that you have a 3.55 rear end. If it was a 3.08, your RPMs would be closer to 1800 and I think its safe to say that 300 RPMs is too big of a margin of error if you were looking at your RPM needle. Also, unless your tires are brand new, they've probably worn down some, which would bring them to more of around 28.5" or 28", which would put you right in the ballpark of 3.55 final drive.

As far as temperature, yeah, -20C is going to have an effect on your gas mileage, even if you warm it up. (Warming it up is sure going to help though!) Mine usually drops a couple mpg in the winter and it usually only gets down to around 15 - 30F (-10 - -1C). I also used to have a car that would get 35mpg in the summer and 17 in the winter.

As always, tune ups are good, but I'd wait until it warms up a bit to make accurate mpg readings.

 

Yeah I'm sure it will be better in the summer. I also noticed a loud grinding noise coming from the front right wheel, I think it might be a bad wheel bearing and the truck starts to shake really bad between 50 and 60 mph but thats a whole new topic lol.

 

Ugg...been there. Wheel bearings are a snap though.