Is the overhead mpg reading really less accurate than a hand calc??

[Gary - K7GLD]

I gave up being bothered that my truck didn't get 50 mpg! Now, with fuel @ $3 a gallon, I just drive until I get 1/2 a tank, then just bit the biggie and fill it back up. If I don't think about it, I use less Prozac! I guess that is one advantage with paying with plastic. You really don't actually see the green leave your wallet until you pay the bill at the end of the month. It also helps that in the past month, Ive only put 200 miles on my truck. If I don't really need to go anywhere, I just stay home. When I do go out, I enjoy the ride and the looks i get with my truck. I guess it just comes down to, you only live once. Enjoy yourself. If you don't spend it now, your kids will the day after you die! That is all for now... ... ... ... ... ... ... I think I will take a ride and rent a video

In the last year, and since relocating here to east Oregon, the truck sees little use. The fuel now in the truck is about 2 months old - I may still be driving on "summer fuel" all winter long!



I hope it doesn't die of old age in the tank!



We mostly drive my wife's '05 Ford Escape - it gets about 26 MPG on the freeway (if we HAD a freeway... ) - and we fill up about every other week on average - it holds 16 gallons.



That's one reason I get sorta cranky when others tell me I should drive less, and slower to "save fuel"...



It isn't that we USE so much that is the issue - it's being ROBBED for what we DO use that burns me up...

 

[MacHaggis]

On the wieght of fuel

In aviation, fuel is gieven as weight for two reasons. First it is useful to know what your plane weighs. Second, and more to the point of our discussion, is that the BTUs in fuel are constant to the weight of the fuel, while the volume varies. If 7-1/2# of fuel occupy one gallon at 60 degrees, it will be more than a gallon at 90, less than a gallon at 30, but will still weigh 7-1/2# and have all the fuel value of the 60-degree gallon.

 

[RedSled]

so based on what macHaggis just said, temperature changes of the fuel can make the hand calculation wrong!!



You cannot fill up your tank twice and use the number of gallons you refill to determine how many gallons you just burned. You could be off by 1 gallon(easily) which would make your hand calcuation off by about . 7mpg



Throw in the fact your odometer is not right and your hand calc is off by at least 1 mpg.

 

[klenger]

Since automotive fuel is usually stored in below ground tanks where the temperature is pretty stable year round, I suspect that temperature does not affect the pumps all that much, but certainly could have some effect.



Gary: Does the overhead computer even have a fuel flow sensor? I've always assumed that the computer just kept track of injection events and durations and calculate fuel flow from that.

 

[Gary - K7GLD]

Gary: Does the overhead computer even have a fuel flow sensor? I've always assumed that the computer just kept track of injection events and durations and calculate fuel flow from that.

I would think the computer is sophisticated enough to calculate the expected volume of fuel per cylinder per revolution, after all. that IS it's primary function where fueling is concerned. What it DOESN'T know, is the precise pop-off pressure of each injector, so actual volume of fuel reaching the combustion chamber can vary, adding to that returned to the fuel tank.



Naturally, add-ons that alter the volume and timing of fuel delivery can totally louse up the MPG calculations of the computer. Either that, or I really AM getting the 26 MPG my OH readout claims!

 

[rweis]

Let's see, I use to be a B-52 pilot.



We could carry maximum about 302,000 # JP4 + ~ 197,000 aircraft = 499,000 total and could go as high as 515,000 in flight in war time as I recall.



@ 6. 4 # / gallon 302,000 # = 47,187. 5 gallons.



My truck gets about 15. 538 (calculated over 3753. 629 gallons) mpg.



15. 538 mpg * 47,187. 5 gallons = 733199. 4 miles, then it is time to fill up again.



Oo.



Bob Weis



Oh, and the government paid for it all too

 

[fkovalski]

In aviation, fuel is gieven as weight for two reasons. First it is useful to know what your plane weighs. Second, and more to the point of our discussion, is that the BTUs in fuel are constant to the weight of the fuel, while the volume varies. If 7-1/2# of fuel occupy one gallon at 60 degrees, it will be more than a gallon at 90, less than a gallon at 30, but will still weigh 7-1/2# and have all the fuel value of the 60-degree gallon.

I'll buy this 'splaination... sort of... . Is not the weight of a gallon of *fluid* constant? (i. e. --water is smething like 7 1/2 lbs per gallon) Maybe diesel is not the same weight as H2O, but prolly close... . so... If fuel (in a plane or truck) weighs 'X' pounds... . cannot that translate to a gallon number? Hence a direct corrolation of 'weight' to 'gallons?'



Your explanation above would mean that the weight to gallon ratio is NOT constant and I am wrong.



-frank.

 

[Gary - K7GLD]

"Your explanation above would mean that the weight to gallon ratio is NOT constant and I am wrong"



No Frank - he said the WEIGHT is constant, but the VOLUME swings with temperature changes...

 

[JVolpe]

Well I have filled mine almost exclusively at the same pump in the same direction and all that BS. I fill to the same basic point everytime. I am sure there is variation in the point at which I stop putting in fuel, but I doubt that it is ever more than one gallon from the last fillup. I reset the overhead after every fillup. I would love for my overhead to be more accurate than my hand calculations, I lately have seen the overhead in the 22. +mpg range. That would be very nice, but my hand calc was 19. 1mpg, which aint bad for me. There are too many things that vary the overhead readout to think it could be anywhere near as accurate as hand calculated. Traffic, variations, ford next to you, wind 0 - 40mph, chevy next to you, temperature changes, ricer next to you, etc. Filling to about the same level and hand calculating will be more accurate for that tank.

 

[RedSled]

right. weight is constant volume changes. as temp of fuel goes up it occupies more space ( and you pay for it, even though you dont get it)\\



what I mean is if it is above 60 degrees you are not getting a true gallon of fuel when you pump. conversely if you pump and the fuel temp is under 60 you are getting a bargain??



I think fuel is sold at a lower temp average in Canada (15 cel). Meaning a gallon is sold at the size (volume) it is when it is 15 cel. what is 15 cel like 40 fahrenheit??

 

[RedSled]

jvlope

what are all those things in your signature? like TST PMCR R49, EZ Stacked, BDDL CL sometimes??

Are those fueling boxes??



If so wouldnt that would make your puters mpg null and void?

 

[ndennis]

fkovalski:"Consistancy is the key. I try to use the same pump on the same low flow fill setting and stop once it shuts off the first time". In my experience with my 04. 5, if I were shut the pump off the first time it shuts off, my truck will be sort 7-10 gallons on fuel. That's right... after shutoff I put in an additional 7-10 gallons. It would not be so bad on figuring mileage if it would consistently shut off at the same point, but these pumps are not consistent in the shutoff point. As much as I use the truck, I can not afford to leave 10 gallons behind. Filling it to the rim is the only way to know consistently how full of fuel you are. So far, I have not experienced any fuel expansion that causes loss of fuel when filled to the rim.

N Dennis

 

[fkovalski]

Filling it to the rim is the only way to know consistently how full of fuel you are. So far, I have not experienced any fuel expansion that causes loss of fuel when filled to the rim.

N Dennis

Yes, this would provide the consistancy to allow one to record gallons accuratly and I am a bit concerned about fuel expansion. I would fill to the brim on long trips and calc my milage but running 75 MPH blows that efficiency. I seem to get better MPG locally because don't have many stops but have to drive gingerly. 1 fill up for me could last a month! But, should I fill right up to the neck?

Great thread!



-frank.

 

[ndennis]

fkovalski: I have owned the following diesel powered vehicles: 1983 D-50; 1987 F-250; 1995 F-250; 1999 VW Golf; 2000 Ram 2500; and 2004. 5 Ram 3500. I always filled these vehcles to the rim with the only expansion overflow was on the 95 Ford and it was a VERY hot day right after fillup. The Golf I was unable to fill up to the rim to my satisfaction... the fuel venting system really made it hard to fill. I routinely drive in areas of 100+ degrees and with the last 2 dodges I have not had expansion overflow due to heat. We are heading into fall, and I would not hesitate to fill er up. Running 75 is hard on the mileage... remember these vehicles have the aerodynamics of a house. The 04. 5 is a good 500 pounds heavier than a 2000 Quadcab and the 04. 5 is 5 inches taller which all adds to the frontal area and drag. On the open road at 2000 rpm (67-68mph), my 04. 5(around 9000 lbs) gets close to 20mpg. My 2000 Ram 2500 gets a little better than 20mpg... but its weight is closer to 6000 pounds... but then again it has an Edge EZ and DDII's and goes like hell.

N Dennis

 

[gocirino]

[fkovalski]Help me out here... . the weight remains constant even though gallons may vary with temperature? ... . But, (not sure what I'm trying to ask. . ) higher temp still equals less gallons even though the Lbs. may be the same for lower temp & more gallons.



fkovalsk

Think more in terms of fuel density. The colder the fuel the more dense it is per gallon thus more energy. The same analogy holds true for a cubic foot of air, the colder one is more dense and has ore molecules per cubic foot.

 

[RedSled]

filling to the rim does not mean you have the same amount of fuel each time.

Here is an slight exageration to show my point





If I fill my tank to the rim, and the fuel is cold. Say the fuel 35 degrees because it was in a tanker truck and its 15 degrees outside and the tanker filled the stations tank this morning. I may be able to get 32 gallons worth of fuel in a tank designed for 30 gallons. This is because the 30 gallon tank rating assumes the fuel is 60 degrees.



conversely as the fuel gets hotter it takes up more space.



filling up at the same station, same direction, holding your mouth the same way etc. when the fuel is hot, will result in less fuel in your tank. You might be able to get 29 gallons worth of fuel in you tank. (even though you just paid for 30 gallons)



Another big reason hand calculations are off, is the fact the odometer is off.

If you odometer is off 3%, which is considered normal, the calcuation will be off by about 1 mpg.



That does not figure in that your tires get smaller as they wear.



That could add to the error, or even correct the error, depending upon which way your odometer is off.



No matter how "careful, or consistant" you are there are other variables that efffect the hand calcuation. It is not perfect. I'm not sure how much "better" it is.

 

[TYCreek]

With energy content being consistent with weight rather than volume ... Now I need to find me a pump station that gives fuel weight! ? ... Then I can calculate MPP (lb) and NMPG (normalized) or at least one that provides average pumped temperature to help calculate TAMPG (TempAdj). Maybe I should add a Price Paid reference and calculate PPM too!



So far, my overhead has been between . 2 & . 8 higher than my calculated MPG, so it seems to me that the averaging algorithms used for the display makes for a pretty good factory gauge. Some of the other gauges I'm not nearly as impressed!

 

[JVolpe]

RedSled,



Yes all of the things in my sig effect the computer overhead, but I ran stock for at least 7,500 miles with only gauges getting a baseline on every thing about the engine. MPG was certainly one of the things I was baselining. TSB 18-037-04 was done while stock except for air intake and exhaust. Overhead was about . 9mpg low before the TSB and 1. 1mpg high. I am not arguing about this, just what I see on this truck. I have been calculating mpg long before there were computers in cars or homes, usually with a pencil and little drivers log book.

 

[RedSled]

tycreek



don't forget you also need to measure your tires, and compensate for the error in your odometer :-laf :-laf

 

[TYCreek]

Maybe one of them wheels strapped to the side too!!! Oo.