In your first post you put in exactly 18 gallons each time (according to your MPG figures). How did you do that? How did you know when it would take exactly 18 gallons? Maybe I'm missing something. If you didn't fill it all the way up each time, how are you figuring your MPG?

 

he said "around or just at 18 gallons"

So technically, if he fills up when the guage is at the same point every time (1/4 left), he should be putting in about the same amount of fuel every time.

 

To calculate fuel mileage correctly you can not go by how much you put in only or using the fuel needle.

First you will never get accurate fuel mileage with just one or two tank fills, you need multiple tank fills, and I would say at least 5 – 10 tank fills to get a decent reading of your actual fuel mileage.

To begin with you fill up until you hear the pump click off. Write down the mileage in a log book. Next time you fill up you do so until the pump clicks off. Write down the mileage and how many gallons were put in. Now also realize that different fuel pumps will click off at different levels (calibration and tolerances). If you really want an accurate reading then always use the same fuel pump.

You then divide the mileage by gallons used (i.e. 18 gallons, 303 miles) so 303/18 = 16.8 miles per gallon. You write down the miles per gallon and do that for 5 – 10 fill ups and then average them all together (i.e. 16.8, 17.2, 17.7, 15.6, 16.5, average = 16.8) You will get a pretty accurate reading of your actual mileage per gallon.

Using the fuel gauge and just putting in 18 gallons is not accurate at all because the fuel gauge is not very accurate. You could have 12 gallons in at one time and the needle registers in the middle, the next time it may only take 10 gallons to have the needle register in the middle.

Therefore never use the fuel needle for any kind of accurate fuel mileage calculations because it will never come out correct…

 

I understand that.

Also, if I let the pump click, Im usually about 3 gallons short...I have to put the rest in myself.

 

He did say that. So technically his MPG calculations are wrong, unless he has figured out a way I have never heard of, that's why I asked.

 

buckdropper, thanks for posting your results and taking the time to test this out.

IMO, a bed cover or change in tailgates can have a minor effect on gas mileage by slightly improving areodynamic drag. However, a 4 mpg difference seems extreme, and I'll bet you would find far less change if you repeat the experiment.

Think of all the things that consume energy when we drive. You've got to spin the motor, the gears, the tires and the accessories. That's lots of friction and tire drag. And the total areodynamic drag is a function of the vehicle's large size as well as the drag coeffiicent, and the truck still has to punch a big hole through the air.

It just doesn't make sense to me that a minor change in the drag coefficient can reduce fuel consumption by over 20% if everything else stays the same. IMO, there are other factors coming into play (like a headwind and bad fuel quality) combining with the changes to the bed.

 

How so?

 

Johngs, read 01XLTSport's post for a detailed explanation. Short version is that you can't just put in the same amount of fuel when your gauge says 1/4 tank or whatever. The only way to accurately do it (that I know of) is to fill it completely to the top and do your calculations from there.

I'm not trying to give Buckdropper a hard time, just pointing out that you can't accurately track your MPG that way. I'm interested in his testing, I would like to see the results after he fills it up each time and calculates it that way.

 

Like I said earlier in this thread, his numbers are not bad, as they agree in large part with my own testing. Going 55 mph on the highway without an air conditioner and in dry conditions, should get you about 21 mpg in a 4.6 with an automatic and a cover. I have done this, and my numbers were with an Extang cover on the back of my truck. If I have to mix in any significant amount of city driving, or any of the other conditions noted above, the mileage will go down fast to about 18, and with legit city driving, in the summer, going 70 when I can, it is more like 16. I have a documented average fuel consumption of 16.4 mpg, which has steadily risen in the past couple of years basically because I started being carefull about accelerations, and staying below 2000 rpm whenever possible at all.

I had the experience of running a number of tanks with and without my cover, and I will stand behind an increase of 1/2 to 1 mpg just due to the cover, and 1/2 to 1 mpg just by never going above 55 on the freeway. My numbers were pretty consistent.

But, this is only for what it is worth. "your mileage may vary" is stated for a good reason...

My 2 cents worth...

TS

 

i used to try and figure out my mileage but gave up after realizing that there are way too many variables to control (traffic, traffic lights, aggressive driving, grade, wind speed, temperature, humidity, and i'm sure theres way more)

i would think that if you had an indoor infinite drag strip, and you could put one gallon of fuel directly to the motor and you could excellerate to 55 mph shifting at the same points you could get a accurate number. barring that its just going to be rough estimates.

some one quite awhile ago posted a topic on this page somewhere. if i remember correctly they took a full size dodge pickup and put it in a wind tunnel.

they tested the air drag on the truck with various setups
hard top, soft top, no top, and with the gate down. i could be wrong, but i seem to remember it had the lowest drag coefficent (sorry about the spelling) with the cover & the gate up.

 

Oh ok. Well I understand that.

But if he goes till about 1/4 left every time, he should be putting in about the same amount of gas every time (if he fills it all the way up).

If he did just put in 18 gallons each time, regardless, then yes, I agree he is doing it wrong.

 

I use to have a 1997 Ford F-150 Supercab Flareside with 3.55 gears and a 4.6 V-8. I would get about 18 MPG on the highway. Now I have a 2003 Ford F-150 Supercrew 4x4 with 3.55 gears, and the 4.6 V-8 (which is way underpowered) and get 13.5 mpg on the highway. That does not make sense. Can someone explain?

Thiis is how I calculate my gas mileage. I zero the odometer, fill the gas tank, drive , stop to fuel up again. Take the milegae off my odometer, and divide that by the amount gas I used to fill my truck. Then I zero the odometer again and start all over.

Heck if I get that kind of gas mileage I might as well get a Hemi Dodge.

 

http://mars.wnec.edu/~ehaffner/did.htm

found this not too long ago..... this was interesting for me to check out. drag coefficients and % increase/decrease in drag tested on a dodge using all types of setups.. square bed cap, aerodynamic cap, tonneau, lowered tailgate, removed tailgate, cargo net, and normal open bed. its cool to see exactly how the air flows over each one of the setups. wish i could find something like this with an f150 used for the test, but either way, it gives a rough idea of what it'll for your gas mileage........

BTW - i got just over 18Mpg off my 2000 s-cab's last tank.. 405 miles and the tank took about 22.3 gallons. not bad for not having a cover on my bed yet, or even an exhaust or intake.....

 

Guys guys remember i am not a professsor of math here this post was just to TEST mileage with the cover off, gate down etc.. i did put only 18 gallons in each time so this weekend i filled it up and gas mileage was 20.39 miles to the gallon (418 miles/20.5 gallons= 20.39 mils per gallon!!( and man its cold). Remember 97 4.6 reg cab with a 5-speed and 3.08 gears. I am not saying it was a perfect test just some headsup knowledge for folks who think a bed cover does or does not help MPG.

 

Ford Sucks! Took my 2003 F-150 Supercrew 4x4 XLT automatic 4.6 V-8 in the dealer yesterday to check out my 13.5 mpg on the highway and was told that is normal for that size engine truck. What a crock. I think I want a Dodge Hemi.