I got a 1998 F-150 x-cab 4x4 4.6. Averages 12 mpg if i run it from full to empty, but if I run it down to half tank and fill up I get 14-15 mpg. If I run it down to 3/4 of a tank I get 17-18. Has anyone hurd of anything this crazy? And does anybody "know" what the problem is. It just dont add up at all.

 

You cannot use the gas gage for mileage checks.

Use the miles driven and gas used to fill up and do the division.

The gage is just fooling you with it's inconsistincy and non linear actions.

 

Bluegrass is right the bottom half of the tank goes faster than the top in these tanks try doing it with the mileage for instance you reset your trip when you fill up and when you fill up again say you drove 300 miles and you put 22 gallons in you get about 13mpg.

miles driven / gallons inserted = MPG

You don't have to fill the tank all the way up to check the mileage.

If i am wrong someone correct me.

 

Well yes you do.... you have to fill the tank to the exact level that it was before you started driving it... that way you know exactly how many gallons you consumed.... i dont think anybody can stop at 3 gallons from the top every time... JMO... but to get an accurate reading of gas milage, you need to fill it up completley and reset the trip odometer...then at your next fill up fill it to the top again and then divide the miles on the odometer by the gallons you filled the truck back up with...

 

There might be slight fuel efficiency differences with varying loads but not that dramatically. If for example you drove most of the time with nearly a full tank of fuel and refueled often, your economy would be slightly lower than if you drove with a low fuel level and refueled when nearly empty, simply because you are carrying a heavier load with nearly a full tank or full.
Otherwise you are likely just being fooled by actual fill levels varied by angle of vehicle at pump and other variables. Fuel economy should be averaged over a number of tanks over time, preferably in the same temperate season. They will always vary due to seasonal changes and fuel blends offered during each season.

 

I stand corrected i always fill my truck up to the top so i guess now that i think of it you can't do it without filling it up all the way without doing alot of math keeping track of previous gallons and mileage and so on so yes the easiest way to check mileage is to fill up all the way.

 

you have to refill each time and divide miles driven by the number of gallons it used since last refill. you do not have to empty the tank each time. you can fill up after using 3/4 tank, etc.... mine gets about 120 miles on the first 1/4 tank, then 210 at 1/2 and 290 at 3/4 and refill at 360 so you see it is not linear.

 

Okay, let me clarify. Here are the miles and gallons and averages that i was talking about. And keep in mind, I topped off the tank everytime and reset the trip meter. This was a trip across Kansas which is practically perfectly flat and straight on I-70. 189miles used 14.95 gal. for 12.65 mpg. then 163 miles used 11.8 gal. for 13.8mpg then 107miles used 6.2 gal for 17.25 mpg. Had the exact same load in the middle of the day. the less miles i drive on a tank of gas the better gas mileage. And if I run the take below 1/4 tank which will be more than 17gal. i get 11mpg if I am lucky. And if I keep the tank above 3/4 I get around 18mpg every fill up no matter temp. And I always use 87 octane. Can afford to use anything else when i only get 11mpg cause i dont want to stop and up $15 in gas 3 times a day to keep it at 17 to 18 mpg. There is a problem and I cant find it.

 

wind... cars in front of you and behind you.. humidity... any of these things can affect milage..

 

Yes, but that was just an example, this is been goin on since i bought the truck a year ago. The lower I go on the fuel gauge the worse gas milage I get. Every fill up everytime. If I fill up and only use 5 gallons give or take I get 18 mpg. If I fill up and used 18 gallons i get 11 mpg. This is everytime. I just wander what could cause it, I dont want to stop at the gas station 5 times a day.

 

its just a little hard to believe Dan. look at it this way, if you drove 100 miles and you got 18 MPG, then drove another 100 and your average for the 200 miles ended up at 12, that means the second 100 miles you got 6MPG, since the first 100 was 18 MPG.--- i think what most guys are trying to say is when you use 5 gallons and refill you dont get mcuh of an average since you can be off a gallon depending on the level of the truck, the temp of the gas, and if it is "really " full. normally you get a better average by refilling when the tank is under 1/4. figure your mileage and do it again next week for several fill ups. one fillup is not much of an average.--- you can also check on the fly. i get about 16-17 mpg on the highway. i know when the needle gets to 1/4 tank i should be at 120 miles, 1/2 tank is 210, 3/4 is 290 miles... i normally refill at about 340 miles per tank. try that.

 

Yes, it is hard to believe, that is why it is the craziest thing I have ever seen. I have done the math 40 or 50 times over the last year. If I burn less than a quarter tank say 6 gallons I get around 108 miles for 18 miles per gallon. And if I burn 15 gallons down to half a tank I get 185 to 190 miles which averages out to 12.5 miles per gallon. The more I drive the worse the fuel milage gets. Hell my 400hp 66 mustang gets better milage with a 289 4brl carb.

 

Dan, I do know that my 99 4.6L Screw's gas guage is late readng for the first 1/3 to 5/8 of a tank. But I still get 16 to 19 MPG.
Are you buying ethenol blended gas? I see a 2 to 4 MPG drop when I run with 10% ethanol fuel. If you are traveling between areas of the nation that are using different fuel mixes that could account for some of the different gas milage at the 3.4 vs 1/2 vs. 1/4 tank that you are writing about. You could also be picking up different octaine gas. And the engine computer is having to adjust the fuel consumption ratio and that could also hurt your MPG's.
Some other questions to ask are: When you drive are you driving long distances or short or a mix?
Does your truck work harder (higher RPMs) the longer you drive it?
Have you had the emissions checked and seen a increase in your emmisions values? My other thought is that your truck is running rich and un-burnt gas is being passed through the emmisions system and being burnt out side the combustion chamber.

 

I dont buy ethenol blends, it hasnt made it to Tennessee yet, were still a bit backwoods for that stuff. I always buy 87, with bad fuel milage and $3 gal for fuel who can afford the higher test, and Im pretty sure the truck calls for just 87 fuel. 95% of my trips in the truck is at least 30 miles highway. But I tested the trucks milage on a trip last week from Colorado to Tennessee around 1200 miles. I was filling up at half a tank 12 to 14 gallons and was getting 13-14 mpg. So I started filling up at 3/4 tank about 8 gallons and was getting close to 17. That got really annoying really fast stoping every 2 hours for gas. I just gave the truck a new tune up about 10k ago. It didnt change the fuel milage. The exaust doesnt smell like it is loading up and since im in TN no one has emissions testing equipment since it isnt required here. Im taking a trip to NC soon, I might stop while im there and get them to put the truck on the vacuum and see what she is reading. What really erks me is a friend of mine has an f250 longbed 4x4 and gets better milage than me. I think there might be an insurance wire getting ready to burn out on the truck. lol