I know there have been a number of posts regarding this subject, but I have just completed my own 'research' on the subject.

To start with, I reset the computer at a fill-up. Filled up till auto shut off, then rounded it to the nearest even amount.

Did the same thing over the course of a month, using the same station and same pump each time. Calculated mileage by hand (okay, used a calculator) each time.

Results -
Computer read between 15.0 and 15.8 each tank
Manual calculations indicated between 15.0 and 15.6

Filled up 6 times over this period, and twice the computer and manual results were exact. Largest discrepancy was 0.3 MPGs

All in all, I would say the computer does a pretty fair job of telling me what my mileage is.

As an fyi, 2003 truck is totally stock, no fuel additives used (although it's starting to get cold so will use them now). Non synthetic oil.

 

I believe if you reset the computer EVERY time, the mileage will check out closer to actual. However, if you do not reset the comuter, it will eventually create a larger spread compared to actual due to the fact that the computer is averaging your fuel economy for the total amount of miles & usage since the last reset.

 

I have a spreadsheet containing every fill up I have ever done, with calculated MPG. The lie-o-meter is usually 1-2.5 MPG off, consistently. After disconnecting the battery, it got worse. My driving is not consistent, but then I'm not sure if that matters. I only use it as a rough gauge. Last time I filled up, it said 18.0 and I calculated just over 16. Granted- I have slightly larger tires, so the error is closer to 1 MPG than 2 on that tank of gas.

 

I have seen similar results with my 04 as Tekrep reported using the same procedure. But if I run say half a tank purely on the highway and run the rest of the tank in the city, it seems as though the actual mileage calculated by hand is about 1 mpg off from the computer.

Here's some food for thought. Assume that you travel 480 miles and get 15 mpg. That would translate to 32 gallons of fuel used. Now if you squeeze an extra gallon of fuel into the tank either to round up to the next round dollar figure or to just top off the tank, your calcuated mileage drops to 14.5 MPG. My point is that very small changes in the amount of fuel you pump translate into larger changes in the calculated fuel mileage.

 

I fill up at a station which, after hours, only allows $60.00 worth of fuel per use. I needed to fill up and only put in the $60.00, (it did not fill all the way up). I noticed my computer showed 20 MPG after about 50 miles on the highway. Prior to this "fill up", it indicated about 17 MPG. Can anybody comment about this?

 

Sure, I'll comment... This is normal, IMO. When you fill up and hit the highway, you are registering only the straight and level MPG you are getting on the highway (including the 10 MPG it was getting when you entered the highway on the on-ramp). Try this- get up to speed, and reset your MPG computer- you will see the "instantaneous" MPG, which will be somewhere in the low 20's if you are straight and level, depending on your altitude and outside temperature. If you run the tank out without resetting the lie-o-meter, you will see very close to your normal average. When you first start out, it is doing its calculation based on the distance you have driven since reset. If you put it in neutral and coast down a 6-mile hill right after filling the tank, you'll see ridiculously high mileage. If you run through 25 gallons of diesel first, it really won't change much. I'm quite the MPG geek...

The bottom line is, if you really want to know how your truck is doing, and want to catch a "trend shift", keep an electronic record (Excel spreadsheet) of all fill-ups. This will show you how it performs towing v. empty, and HWY v. City. It will also let you know if something has REALLY changed- many people think they are getting bad mileage, but it is a function of condition and driver input.

 

I have been waiting to post on this same subject. The last 4 tanks where I have been tracking the mileage has shown the readout to be within 2 tenths every time. I wonder why some seem to work and others do not?

 

I have also found that the the lie-o-meter is optimistic (by 1~2 mpg). I top off every tank, and reset the computer every tank. On my spreadsheet, I added a column that shows what the computer showed as my mpg vs. my actual hand-calculated mpg. Seems relatively consistent in terms of how much it is off from tank to tank. My truck is stock.

 

Don't know either but I have had the same exact results on 3 consecutive fill-up. Procedure is: Reset the lie-o-meter, reset the trip odometer and use the hand calculated method.........l and both agree to 1/10th on all three. Bizarre but exciting!!! The only mechanical mod on the truck is the EGR disconnect. Stock tires and rims and no change to the drivetrain. Will keep you posted!!!

Claude

 

Like a lot of you folks, I keep exact records, and fill to top of the filler tube each fillup. I reset the overhead each time and hand calculate. My overhead is always 1+ gallon better than "real".

 

I just got back from a quick week trip to the dark, cold, damp, green town of Enumclaw WA to visit the kids and used the opportunity to track my mileage. Going over unplowed Chinook pass in the fog was a thrill I would rather not repeat. I did NOT reset the computer until my final fill and it seems to have tracked within 3% of the calculated mileage.
2839 Miles
165.0 Gals
17.2 MPG Calculated
17.8 MPG Computer
This seems close enough to use as a reference for the same type of driving.
As another interesting check I had my GPS on to check cumlative miles and speed. The GPS actually showed 2851 miles, so the Ford odometer seems pretty close. The speedometer varied, but always indicated one or two MPH faster than the GPS. This was good for me, as I like to set the cruise for 5 MPH more than posted .

Wayne

 

Reset mine every fill up, fill to the neck, no mods. Approx 1/2 mpg off each time.

You guys should see my mpg speadsheet!

 

Does anyone think that there is a correlation between tank size (shortbed vs longbed) and the variation of the overhead and actual? Mine is a shortbed and the the overhead is consistently 1+ mpg higher than actual.

This may be valid only for those who fill to the top of the filler neck.

 

Mine is a 44 gallon, and it is within .6 to 1.1 higher all the time

 

I have noticed that if I fill mine to the neck that the lie-o-meter reads higher than the calculated. I have assumed that this had something to do with the fact that I didn't fill to the top of the filler neck on the previous fill up. I'll have to try filling to the top of the filler neck for several fill ups to see if it makes a difference.