So I used to let it run down until the light came on and then go another 30+ miles. I was complaining about MPGs on here and was informed I should not let the tank get so low.
Previously I was getting 15 or less MPG Last time I filled up just below half and it was 17mpg not sure why that is but I'll take it, thought I'd share.

 

it don't matter what I drive. when winter blend gas comes out, my mileage drops. I just filled the Durango a couple hours ago... O/H console says 10.8. ridiculous. usually its closer to 13 with summer blend without much highway time. and I rarely drive it, if I did my mileage would be that poor all the time. but my wife drives this one. makes the MPG that much more abysmal.... I got my best from this Durango on a trip to texas about 5 years ago, just over 20. all highway miles. late spring/early summer.

 

I forgot to mention its the 94, 3.9L, in the last 6 years- Plenum kit, new exhaust (Repaired broken exhaust manifold studs), timing chain and tensioner, new distributor, all new sensors, new battery, regular maintenance and just recently replaced all of the fuses in the cab fuse box.And a rebuilt trans.It got <9 before that. When I first bought it the gas gauge did not work I filled it and when I got to 150 miles on the trip ODO (22gal tank) it ran out of gas. That's 6.81 MPG.

 

Though the engine is a dog, and I have gotten better mileage with a 318 i have never gotten anywhere near that bad out of any 3.9. my 90 dakota with the pre magnum version was the best in that department, pretty consistent 16 in town, 20 on a all highway run.
​that was a reg cab long box (yeah 8ft)
then I had a 94 dakota reg cab short box, that 3.9 was good for 14 in town, 17 on the road, best it ever got. My current 99 dakota gets a pretty steady 17. All with 3.55 gears.
Then you have my 93 318 dakota club cab that I bought with a dead trans, I converted to a NV 3500, stick. That truck would get 22 on the road.rarely below 17 no matter how or where I drove it. I miss that truck.it was in a horrible wreck. Currently I have the 99 and a 96 4wd club cab with a 318 and 3.91 gears, I get 14-17 from this one. With lower gear and the extra weight of 4wd so while not horrible from a 3.9 but if I can be getting +/- a couple of miles per gallon and have a V8 with more power and less sogginess give me the V8. I've typically bought what was available at any given time, v6 or v8, I'd rather have a V8.

Now having said that my worst was an 88 D150 with a 318 tbi which was the 1st year of that horrible system. With a 40 mph tailwind I could get 12 out of, on a long highway run.its usual was 8. It ran fine but was just thirsty. And you talk about good money after bad to try to improve that number, it still pisses me off to remember that truck.my parents big block plymouth wagons did better in the 70s and early 80s than that.
I've had such vehicles as a 78 D300 duallie with a 360 and 4 speed/granny low 1st that would get 10 no matter what, and an 86 dodge van with a 318 that would get me 16 in town and 19 on the road with ac cranking, 21 when not running the ac. Though I don't buy trucks for top mileage, or if be buying a Prius, I try to get them to get the best they can do for what they are. Sounds like your truck must have been a Monday morning build.
no flash required or even available, but maybe try plugging in another computer brain. I had an old diplomat ex cop car that I had 2 computers for it would run better in warm months on one of them and better in winter with the other one. I had that one a long time and yeah once I found this out I did swap them for the season.

 

I may be missing something, but what difference does it make if you fill up at empty or 1/4 tank? I would think it would be, fill the truck up, write down the mileage, run the truck, fill up again, and write down the mileage. Take the difference in mileage ie. 250 miles, and divide it by the gals. purchased the second time. ie 250 miles traveled, purchased 10 gals. so mileage would be 25 mpg.

Also, what is the difference in winter blend and summer blend? At least in my area its all 10% alc. .... Again, im asking is there a difference, and there may be......

 

Winter/Summer gas use different additive packages, I don't know exactly what they consist of, but, I know ALL of my vehicles get worse gas mileage in winter, and my driving style really doesn't change. I don't warm them up, I go out, get in, start it up, drive it away. Same as in summer.

 

That is what I thought too. I ran it down as close to bottom as I felt I could trying to get a reliable measure of how far I could go on a tank without filling. My goal is to get 400 miles on the 22 gallons. I'm not even close. First tank after the plenum job I got 340miles and still had gas. Now it gets up to 320 and no more. but I digress.....
If I remember right was told that the fuel pump works harder as it gets lower because the gas itself is the cooling agent for it and that has something to do with mileage. It seems big jump to get 17mpg at just below half fill up and 15.5 or less when I go down until the needle is past the bottom line.
On winter gas at a guess the additives are to make it more easily vaporize. At a guess that would probably mean those things evaporate more quickly so you lose that by evaporation and then the extra gas that has to be pumped as the additive VOC's or evaporated off.

 

Quote:

Originally Posted by HeyYou

Winter/Summer gas use different additive packages, I don't know exactly what they consist of, but, I know ALL of my vehicles get worse gas mileage in winter, and my driving style really doesn't change. I don't warm them up, I go out, get in, start it up, drive it away. Same as in summer.

I feel sure you are correct with something different in the packages for winter. But in addition, winter driving is going to be worse due to it taking longer to warm the motor,(regardless of no warm up prior to driving) as well as the diff, and trans. oils..... And of course, the shorter the commute, the more effect it will have on mpg.

 

Quote:

Originally Posted by onemore94dak

That is what I thought too. I ran it down as close to bottom as I felt I could trying to get a reliable measure of how far I could go on a tank without filling. My goal is to get 400 miles on the 22 gallons. I'm not even close. First tank after the plenum job I got 340miles and still had gas. Now it gets up to 320 and no more. but I digress.....
If I remember right was told that the fuel pump works harder as it gets lower because the gas itself is the cooling agent for it and that has something to do with mileage. It seems big jump to get 17mpg at just below half fill up and 15.5 or less when I go down until the needle is past the bottom line.
On winter gas at a guess the additives are to make it more easily vaporize. At a guess that would probably mean those things evaporate more quickly so you lose that by evaporation and then the extra gas that has to be pumped as the additive VOC's or evaporated off.

Just to be sure, are you filling the truck up, then recording the mileage traveled, filling it up again, then dividing the miles traveled by the gals. purchased? Its the only way to accurately check mpg. The guage is not accurate enough to look at....

As far as running the truck down to empty..... I find it hard to imagine how it could effect gas mileage. I have heard rumors of overheating the pump, but most of those claims are due to it not being submerged in gas. And JMO but I find those hard to believe for several reasons..... If it was true, the manufacture would warn against it, Also, many vehicles have external pumps and some even dead head with no return....

As far as evaporation goes, you have a sealed system, that collects the fumes and burns them..... In addition, especially in a MPI system the gas is injected as a liquid in the combustion port, and then evaporated. The heat in there would be pretty much the same in winter or summer, at least I would think....

 

Yup. I use the trip odometer and divide by gallons to fill up.
I was just guessing as to why the winter blend gave worse mileage.