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

[MacHaggis]

... but I think we're already WAY out of scope on this thread to continue! I'm not aware of a pressure/volume/temperature relationship for liquids that's as convenient as the ideal gas law is for gasses... what's known as the "Tds" equations might be the only way to work temperature and volume into the mix for liquids. Again, WAY out of scope!





-Ryan

I certainly agree on the scope thing, and the hydrostatic equation being more applicable. I fell into the trap of using the direct relationship of Boyle as an illustration of the concept that wouldn't get into differential calculus.



I did put the question out to a couple of engineer friends asking for some insight. If I get anything useful back, I'll PM you.



Now, hasn't this been fun? :-laf



Scotty

 

[rbattelle]

Now, hasn't this been fun? :-laf

And remarkably no one's got angry... yet.



-Ryan

 

[klenger]

Angry ... no, bored ... yes.

 

[BIG BOB]

I acutally filled up today with this in mind and I must say my tank was "full to the rim" for almost 2 gallons. I could fill it up until it was showing at the neck, then wait two minutes and it would go down. after 10 minutes of this I guess it was full, but honestly i didn't wait 2 more minutes to make sure. I added 2 gallons after it was "full to the rim" the first time

One thing I haven't see mentioned is the "Spooled Up's fuel tank vent kit ". This kit allow you to fill your tank all the way into the neck if you want, in about a minute after the automatic shut off goes off. I always fill mine to the neck, and it will not go down, it is all the way full. Then I hand calc mileage, for accuracy.

I know that before I installed this kit, I could baby feed about 4 to 5 more gallons into the fuel take. It would take about 10+ minutes to do it. So if you haven't had this kit installed you are probably are not filling your tank, let alone to the same level every time, so your mileage will not be accurate.



One other thing, 5 years on this board, many posts about the overhead has confirmed that it isn't accurate. Rather than spending your time arguing, do some searches of former post, the results are there.

 

[Mrudolph]

I don't care witch method is correct (and I track both), when the mpg goes up I can drive more miles for less yankee peso's. My overhead is consistanly 1. 5 - 2 mpg less than hand calc's, maybe my pen is broken?

 

[Giachino]

I've done a few hand calcs and compated vs the computer... .computers . 5 high..... at this point, what's the point of tracking mileage? Is it gonna get better if I track it? IMO the computer is accurate enough to know if you have any mechanical issues... . becasue it will swing one way or another vs the norm..... so who really cares what the exact mpg is once you know your baseline? Sure, if you change tires, or do other mods, I could see going through the exercise again to determine your new baseline... . but assuming the truck remains the same, and is broken in, why would i waste my time hand calculating my mpg for 300000 miles? Am i missing something?

 

[klenger]

I wanted to add one more observation that I have noticed the the O/H computer. If I fuel up and then drive a partial tank towing, then the rest of the tank driving empty, the O/H gets confused and tends to skew towards the initial results. Same thing happens if I drive empty and then tow for the later part of the tank.

 

[CKimball]

heck... i should have paid attention in school instead of dreamin about cars and chasing the chicks... . that said... nailed lots of broads and own lots of cars and still have 1/2 a brain... (surprizing as I grew up at the beach and went to "tubin and Doobin high!!!!")... great thread guys!

cameron

 

[JVolpe]

And remarkably no one's got angry... yet.



-Ryan

Not sure I have seen a thread this long without a lot of negative innuendo and specific well aimed pot shots before. Congratulations to everyone Oo. .



I am getting a headache from thinking about differential calculus and such things that I have been doing my best to forget for over 30 year now.



And after reading every d@m post I still believe that consistent repetitive hand calculation is more accurate than my overhead computer readout.

 

[bighammer]

Not sure I have seen a thread this long without a lot of negative innuendo and specific well aimed pot shots before. Congratulations to everyone Oo.

Makes for a lot of reading for the moderator too. Whew

 

[Gary - K7GLD]

I wanted to add one more observation that I have noticed the the O/H computer. If I fuel up and then drive a partial tank towing, then the rest of the tank driving empty, the O/H gets confused and tends to skew towards the initial results. Same thing happens if I drive empty and then tow for the later part of the tank.

You have to realize the OH computer averages fuel consumption - as I understand it, the 2nd generation truck averaged over the last 300 miles driven for displayed MPG, and the 3rd generation trucks over the last 200 miles.



In any event, it will take a while in extreme driving situations for the OH to reach a representative average - such as going from 100 miles of heavy towing, over to the next 100 miles driving empty and unloaded.

 

[klenger]

Gary: I just assumed that it averaged it since the last reset, regardless of how many miles it's been. If you are correct, then comparing the O/H to calculated MPG is pretty meaningless if the driving conditions vary, which means that the 5 pages of this tread are meaningless.

 

[Gary - K7GLD]

Gary: I just assumed that it averaged it since the last reset, regardless of how many miles it's been. If you are correct, then comparing the O/H to calculated MPG is pretty meaningless if the driving conditions vary, which means that the 5 pages of this tread are meaningless.

The only way the OH is of value for checking MPG, is to reset it when you fillup, and then check the displayed MPG at the next fillup, as long as it's within that 300/200 mile range.



At least that's the way I understand the average MPG feature - there's a remote chance someone will correct me if I'm wrong...

 

[klenger]

:-laf

there's a remote chance someone will correct me if I'm wrong...

Not correct. There is a very good chance someone will correct you if you are wrong. After all, this is a truck forum.

 

[crobertson1]

Could one of you guys with a much better understanding of how this OH thing works help me? I always wonder why some guys have DTE readings in the 500s when filled. Mine is usually around 390 miles after a fill up. Does it have to do with the fact that I mostly use this truck for towing and the OH has been acclimated to less MPG? So, then after driving empty for a while, the MPG goes up to 16. 5 or so, the DTE still reads low when full. For instance, I had driven unloaded and the mpg had raised to about 16. 7 and I filled up. I figured that at 16. 7 mpg and at least 34 gallons in the tank, the DTE ought to be at like 567. It wasn't. It said 390.

 

[Gary - K7GLD]

:-laf



Not correct. There is a very good chance someone will correct you if you are wrong. After all, this is a truck forum.

Shucks, I don't even need to BE wrong to get corrected - that was the reason for the smilies...

 

[BigGunZ]

RedSled,

Having read your initial post, you're absolutely correct in the sense that...

No matter which method we're using to compute MPG's, we're doing so using data from some sensor on the truck. Lets face it guys... we're not exactly talking "space shuttle" quality sensors here. They're off... . they deviate... . and in every sense, affect the accuracy of your calculations, sometimes by large margins.

Additionally, another member (don't remember who) with an aviation background, raises valid issues as well, specifically concerning specific gravity or weight. A common jet fuel (which is very similar to Diesel), called JP-5 weighs approx. 6. 8 lbs per gallon during what is referred to as standard day". I f I remember correctly, standard day is 59 deg. F at Sea level. Deviation from those exact conditions change the density of the fuel and inevitably, your fuel load (quantity), and mileage.

Bottom line... .
There is no accurate way except hand calculations accounting for the specific gravity of the fuel, and acounting for the current atmospheric conditions. I highly doubt anyone does that and if so, you're still off since there's no accurate way to measure fuel flow.

Me... I take my hand calculations and average them with the overhead. In reality, hand calc's read slightly low and the overhead reads slighty high. Average the 2 and you're likely close.

 

[Elite1]

When my 03' was completely stock, I checked it by hand & the overhead mpg seemed to be very accurate. But when I put my "Volumizer" electronic box on, it started reading that I was getting incredible mileage, about 28 mpg on the highway... when checked by hand it was way off, I am actually getting about 20 or 21 mpg if I baby her and keep my foot out of the throttle which is hard to do!