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

[RedSled]

I know the opinion of many on this web site is the only accurate measurement of mpg is by hand, but I am not so sure of that



many will disagree, but I think my overhead is more accurate. Hand Calculating can be flawed unless you know a couple of things.



If I calculate by hand it is usually within . 5 mpg of the overhead, and I think the overhead is right. here is why.



1) A hand calcuation is dependent upon the fact that you can fill the tank twice to the exact same amount.



If you are off by 1 gallon either way that will skew your calcuation.



say you fill up and put in 29 gallons then drive 600 miles. Then you fill up with 30 gallons. what is your gas mileage? based on a hand calcuation you would say it is 20. 0 but is is really 20. 7. U



unless you fill up the same both times your hand calculation is wrong by . 7 mpg. can you really tell if you are off by 1 gallon?



I dont think the shutoff valve on a gas pump is that accurate to say they all shut off at the same point.



I have used a few that don't shut off until diesel is spewing out of the filler neck. I have used some that I can add 3 more gallons after the first shut off, and some that I can only add another gallon. If you fill up each time until it shuts off 3 times(even on the same pump) how do you know your not off by 1 gallon. One batch of diesel may foam more than another.







2) an accurate hand calculation is dependent upon the odometer being correct.



a quick search on the web shows that odometers have an average 1-2% margin of error. sample site



Tire wear will increase that figure by another 1-2%. Point is your odometer could easily be off by 4%. This fact alone would result in the hand calcualtion being off by . 8 mpg from acutual mpg for example:



Say you actually drove 600 miles, but your odometer had a 4% error:



624 miles (+ 4% error) / 30 gallons = 20. 8 mpg

600 miles (accurate odometer) / 30 gallons = 20. 0 mpg

576 miles (- 4% error) / 30 gallons = 19. 2



3) if you combine the two scenarios above you could be off by 1-2 mpg



filled tank with 29 gallons drove 600 miles, then filled up with 30 gallons, but you odometer is 4% off. (odometer says you went 576 miles)



a hand calcuation would go like this: 576 miles / 30 gallons at fill up ( you would not know you only had 29 in the tank). = 19. 2 mpg



but the real calculation should be 600 miles / 29 gallons = 20. 8 mpg



that is a difference of 1. 6 mpg



So exactly how does the trip computer work to calcuate mileage? Why do so many people comment that it is junk? Is it only inaccurate when you modify the fuel delivery system with a box?

 

[JVolpe]

YES! You can fill the tank to the neck each time and get a much more accurate calculation.

 

[rweis]

I've been calculating mine since mile #419 (I bought it wilt 418 miles on it).



I would bet hand calculating it the past 59013 would be fairly accurate. My spreadsheet just keeps adding up the miles and the gallons and the $$.



Bob Weis

 

[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. Even using these methods, I am still off by about 1. 5- 2 MPG from hand calc to OH comp. I wish they were closer just to know which one is correct. I am tending to favor MY hand calc are more accurate. Hand= 15. 6..... OH= 17. 0 mpg)



I think it is a conspiacy on DC's part to "build- in" a higher mpg factor of say 2- 3 mpg to give the ignoramus the warm and fuzzy's to brag at how good a milage his truck is getting <---half serious on that last on!



Of course if one were to keep long term records you will get accurate overall calculations of gallon used over "x" miles.

 

[Hohn]

Exactly. Consistency is EVERYTHING. If you can use the same pump at the same station with your truck parked at the same direction, angle, inclination, etc, then you can all but guarantee that you are getting VERY consistent fill-ups.

Failing to do the above may, in fact, cause hand calcs to be off-- maybe even worse than OH. Of course, you bomb the engine at all, and OH is out the window!

I don't have an OH, so that simplifies things

 

[RedSled]

Temperature can also impact the calculation

Here is a portion of an article that adresses the impact the temperature of the fuel as you pump it.



Basically this article would imply even if you use the same pump, fill it up consistantly etc. the temperature of the fuel will create a 1-3% margin of error. The 30 gallons you pumped last week at 90 degrees takes up more space than the 30 gallons at 60 degrees. (say the diesel was 90 degrees when you filled up last week because it was fresh out of the tanker truck. When you filled up this week it had cooled to 60 degrees because it has been sitting under ground for 7 days. ) Your calculation could be off by 1 mpg based on the difference in volume caused by the difference in temperature.





Whether you’re pumping diesel or gasoline, pretty much expect to get what you pay for. You do – only if the fuel coming out of that nozzle is 60 degrees. Anything warmer than that, and you’re getting cheated.

Fluids expand as they warm and contract when they cool. Diesel and gasoline are no exception. How much? Estimates vary, on average from 1 percent to 3 percent just for diesel, depending on how hot it is and whom you ask.

So if that gasoline or diesel you just pumped into your car or big rig is hotter than 60 degrees, your gallon just got “bigger. ”



Is bigger really better?

Imagine this: Btu units are marbles filling a gallon glass jar surrounded by diesel or gas. When fuel is warmed, the Btu – marbles – separate from each other half an inch, and some are forced out the top of the jar. The jar has the same volume – it still measures out at a gallon – but has less Btu “marbles” to burn.

If that fuel you just pumped into your rig is 70 or 80 degrees – it’s basically weaker. It doesn’t have the same burn for the buck that it would have had at 60 degrees.

If you consider those two things – Btu content and volume – together, then you will see the problem. This is a perfect example of where the “bigger isn’t always better” adage proves true.



The magic number

Because the amount of energy per gallon varies with its temperature, engineers need a baseline for measuring and calculating. Diesel and gasoline volumes at the wholesale level are standardized to 60 degrees – the magic number – which is about the temperature of old single-walled, underground storage tanks. At 12 feet deep, the contents of those tanks are kept a relatively constant temperature throughout the year.

Today’s double-walled tanks keep fuel near the temperature it was when it was delivered. They act just like a thermos, and if the fuel doesn’t stay long in the tank, it can be so warm as to be labeled “hot. ”

During the summer months especially, fuel can be delivered to a tank at a temperature that is nearly 40 degrees hotter than the benchmark used at the wholesale level. So if the fuel’s temperature is not compensated for at the truck stop pump, the gallon pumped into your tank will be less dense – with fewer Btu – therefore less powerful.

During the summer months, an informal OOIDA Foundation study of 32 fill-ups around the country found the coolest fuel at 71 degrees. The hottest fuel was 98 degrees.

 

[klenger]

Overhead milage readings are for entertainment purposes only. It's the fuel that you put in the tank that cost $$$, and that's all that matters.

 

[AJain]

Overhead mpg readings

My overhead CONSISTENTLY indicates about 1. 6 mpg higher than hand calculated. And that is with recalibrated ECM wheel revs which check to within 0. 5 mph or less of my speed as indicated on a GPS. Although, I haven't yet compared the GPS distance travelled with the odometer over a long distance, which would be the most accurate way to go.



This is an interesting topic, as I've also wondered how this is calculated by the computer. MPG = miles driven / gallons used. So assuming that the odometer is accurate, an inaccurate overhead mpg means that it is inaccurate in assessing the amount of fuel used.



When my tank gets below 1/8 full, I switch to the DTE display to monitor that while driving. Once it drops below an indicated DTE of about 100, it begins to "catch up". It drops from a DTE 100 to a DTE of 0 in way less than 100 miles. Obviously, the computer is revising the quantity of fuel it "thinks" is still on board as the tank nears empty. One would think that this should also change (drop) the indicated MPG to account for this correction, but for some reason it doesn't seem to affect it. (I only reset my MPG when I fill up. I will have to try resetting it at about 100 DTE and see what it does in just this range... )



I think they just use a float sensor to figure out how much gas is in the tank. Generally, there is a lot of inherent inaccuracy in a float-based indicator system. Totalizers are the only way to go for real accuracy.



aJ

 

[GFrance]

RedSled raised an interesting point to consider when hand calaulating. When I deliver fuel to a station we bill that station on temperature corrected gallons. Temperature & gravity of the product has an effect on the total gallons droped into the tanks. Difference between gross gallons delivered & net gallons delivered can be 100 gallons diff. on a 11,600 gallon load. In the summertime & warm climate areas the station is billed for more gallons (expansion) the other way around in winter & cold climates. Here this time of year the gallons are almost equel between net & gross.

 

[RedSled]

gfrance,



I think you see exactly what i am trying to say,

hand calculations have a margin of error based on several things. The simple examples I gave all combined show the hand calculations can be off anywhere from 1-2 mpg even if you are very careful.



Without knowing how the computer calculates mpg it is an assumption to say hand calculations are better. Maybe the same variables effect the computer, maybe not. maybe, just maybe the overhead is more accurate.

 

[RedSled]

klenger,



I dont understand what you mean. what does the cost of fuel have to do with the accuracy of calculating mpg???

 

[gocirino]

Fuel temperature is why we use pounds and not gallons in jet aircraft. Even the fuel flowmeters are in pounds per hour..... gallons are just too inaccurate!

 

[fkovalski]

Fuel temperature is why we use pounds and not gallons in jet aircraft. Even the fuel flowmeters are in pounds per hour..... gallons are just too inaccurate!

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.



~ even though weight may be the same do you really have the same *AMOUNT* of fuel?... <--that's what i'm trying to understand.

 

[klenger]

klenger,



I dont understand what you mean. what does the cost of fuel have to do with the accuracy of calculating mpg???

Cost of fuel is the main reason we even care about milage. If fuel was free, we wouldn't even care what our milage is. How many gallons miles do we get per gallon of air consumed by our trucks. What, you don't know? Probably because it's free (so to speak), so we don't track it.



If my overhead says I'm getting 50 MPG, but my hand calculations tell me I'm getting 20 MPG, then I'm paying for fuel at 20 MPG, not 50.

 

[RedSled]

klenger,



you just stated my point, sort of, but in reverse. The hand calcuations are subject to errors. if your hand calculation shows your getting 20 mpg it could be wrong by 1-2 mpg due to the variables already mentioned.



If your computer says your getting 21 mpg, and your hand calc says 19. 5 mpg you cant say for sure which is more accurate.



now if you hand calc says 20, and the ovehead says 50, well you have issues with the computer.

 

[klenger]

You're still missing my point. The only milage that matters is the fuel I put in my tank at the pump, regardless of what the overhead says. I've tracked all of my fuel use since I got the truck 20,000 miles ago. Any variations in fueling average out over a few tanks. BTW, my overhead usually matches my calculations to within about 0. 5 MPG.

 

[rbattelle]

This is why I carefully calculate fuel mileage on every tank, then watch only for large deviations from the mean.



-Ryan

 

[jwduke]

Mine has been off 1. 5 - 2 mpg since new. I think the thermometer is more accurate!

 

[Gary - K7GLD]

ONE reason the computer can't accurately display MPG, is that there are no sensors in the fuel return line back to the fuel tank - fuel that was NOT burned within any given point in time.



Dunno about other computer MPG readouts, but the one on my '85 vette was dead-on accurate, because EACH individual fuel injection event was controlled by the computer, there was NO bypassed fuel or other "slippage" to confuse or slew calculations.



I would think the new HPCR fuel setups would also be dead on, since there too, each individual injection event is fully computed and controlled by the computer, and fuel returned to the tank doesn't figure into the calculation.

 

[2Gen3Gen]

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 bite 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. Then, i just hit the "pay it now" option on the computer and the bank does the rest! 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