All right, let's just get it out of the way now, this sounds ridiculous. I'm a moron; I don't know how to fill a tank; I'm reading the gas gauge wrong; my calculator isn't working right. Whatever, but here it is.

When I fill up from about half tank I get a little better gas mileage than if I fill up from empty. That is, after I've filled the tank and I only dispensed about 8 or 9 gallons and do the math, I will get about 19-20 miles per gallon on the highway. If I fill up and dispense about 20+ gallons I will only get about 17-18 miles per gallon. While the numbers are little lower, I experience the same difference while city driving.

I've noticed this since the truck was new in '04 but didn't say anything about it because it didn't make sense and, frankly, I was just a bit embarrassed to mention it. Now I see on another forum (Dodge trucks) that someone brought up something similar.

Anyone else experience anything like this? If I'm the only one I'll walk away quietly and never bring it up again. If I'm not the only one, does anyone have an explanation?

Dave

 

Hmmm, that is interesting. I trust your math because I know people tend to knock on it for an explanation which should not be the case. Will be intersting to see if others have had this happen.

 

The EVAP only runs if you are in the 1/4-3/4 full range, so if you always were above 3/4 full you'd get the best mileage possible, but you'd have to stop every 75-100 miles

 

i get good mileage on the first 1/4 to 1/2 tank, but then it drops off kinda fast. i think its just the gauge.

 

I'll be honest, my longest is the 1/4 then the quarter between 1/2 and 1/4 LITERALLY lasts 20 miles. First last around 100-120 then the 3/4 to 1/4 last around 100 then the last 1/4 last around 100-120

 

I think the OP is referring to his actual MPG numbers, not how fast the gauge moves. Patman, above, seems to have a plausible answer as to why the mileage is better for the top half of the tank.

 

Oh I got you, my actual MPG numbers suck too haha

 

That must stink

 

mtylerb:
You're exactly right. I am referring to the actual mileage I calculate after I fill the tank. I divide the number of miles driven by the number of gallons required to bring the tank to full. By full I mean the pumps shuts off by itself and I round up to a even number.

I could dismiss this whole thing if it was just once or twice, figuring I just "topped off" differently. But this happens practically every time and it's been going on for about five years.

Just so you know, I'm about 50 years old, educated and have never been abducted by aliens. Of course, when I tell my dad about this he thinks I'm a bloomin' idiot.

Patman may be right. I don't know that much about the EVAP - yet.

Dave

 

My mileage stays fairly consistant. Well, except for one blip in April that hit 28 MPG. Not entirely sure what happened there, but I think it had to do with the tank overfilling. Keep in mind the prices below are in CAD and I'm filling up in Litres, not Gallons.

Mileage:


April was mostly highway driving. The rest has been majority city driving and only about 30 km's per day.

 

What's an EVAP?

 

Well Gray Scab, you're not crazy. However an explanation is difficult to formulate. As a fellow non-alien abductee (I actually laughed out loud when I read that) I'll take a shot at this. The only thing I can possibly think of is there could be a little debris in your tank. When the tank is full the debris is less likely to be picked up by the fuel sending unit. As the tank volume decreases the debris starts to get picked up by the sending unit and adversely impacts your fuel economy.

I suggest changing your fuel filter and seeing if this helps. For the time and money it certainly can't hurt. Should take you about 10 minutes.

This phenomenon certainly is counter intuitive. One would think that as the tank level drops the weight of the vehicle also drops and fuel economy should go up. Going from memory but a rule of thumb is for every 100 pounds you carry around doesn't it hit your fuel economy by 0.1 MPG? Fuel certainly isn't as heavy as water but a 27 or 36 gallon tank sure is heavy and once you get rid of some weight it would be reasonable to think that it is "easier" for the truck to propel itself.

Quite a mind-bender isn't it?

 

I can't think of a logical reason for a difference in mileage other than the obvious weight difference, and even that's counter-intuitive as you're getting better mileage when it's heavier. The only logical reason I can think of is the less you fill, the more chance of error coming into play. A variation of 1 quart based on 5 gallons would be more significant than 1 quart based on 15 gallons. I know it's a longshot but it's all I can come up with.

Best suggestion I could give is try filling at the same station, same pump, same time of day, maybe even the same day of the week in case there's something going on there. You've removed as many variables as you can, only ones left are service station tank temp, level and any potential variations in the pump shutoff based on unknowns.

If it still holds true, then fill more often for that extra mpg. I think it could be worth it, but I have no other idea why it might be happening.

Paul

 

You drive a **** load!

 

You are not crazy and you are correct. I posted this a long time ago and got laughed at. I have kept a spreadsheet on mileage since my truck was new, 4 years. Everytime I take a trip, I will go top the tank off before I head out. If I put in less than half a tank, I get much better mileage. Over 1/2 mpg better. I dont have any explanation other than evaporation, but I know it is real and a fact. It happens every time.