Overhead Computer.. total fiction

[PJ]

Right now, my over head states 15. 7mpg, yet I just filled up at the SAME station and did the math myself. I got 13. 1mpg. This is round town driving which = roads of 45-55 mph with little to no stop and go driving. I did a drive north on I-5 for about a 150 mile round trip and got a reading above of 18. 9 and did the math at 17. 0. All i ask is why is it such a deal for a "few" guys here to claim 22=mpg at 80+ mph??? Im sorry but that is just a dream in some of these guys heads. There is NO WAY that a truck of this size and weight that is totally stock get those figures. My dealers even say they have never seen ANYONE pull these figures. . Its like myself getting into my Corvette and claiming to get 45 mpg at 95mph. The only way that would happen is on the down hill side of I-5 with the wind behind me and the motor OFF. Come on guys. Lets check and then re check your fiugures as I really fine it very hard to believe the figures that are being posted. I see guys with the very same equipped truck getting 5 - 8 mpg diff reading from the other guy with the same driving cond. Whats wrong with this picture>??? I think Maybe I should just start telling I get a flat 20mpg at any speed and leave it at that. Lets just all get real on this issue. Im not wanting to flame anyone on this subject but... . lets just report the real facts. and be correct! I must say that my 04 305/555 did in fact get about 3 mpg more than my new truck but it was a 2500 without anti spin. other than that, it was the same truck, but... It had been waxed so maybe the wind had something to do with that.

 

[1Tufdsl]

Not sure why yours if off by so much. My overhead computer is spot on everytime I hand calc my mileage. When I hand calc I make sure I go to the same pump even and fill up as full as I possibly can. When I do that there is no discrepency in the O/H comp.



There are so many factors in mileage it's hard to compare apples to apples. The best I've ever gotten on mine is on flat ground going through town on the freeway at 60 mph after just fueling up and seeing 20. 3mpg. Not bad for nearly "4 tons of fun" rolling down the road. But I'm sure some do embellish a bit. Kind of like than pan sized trout that gets to be a 4 pounder by the time you're back home telling your buddies about it.

 

[Spooled-up]

I just posted this in another thread, but it relates to this one:



Even when you hand calculate, if you don't fill up so you can see fuel in the filler neck, you could be off by quite a bit. There is a significant air space in the rear of the tank that you can fill up by slowly trickling fuel into the neck. If you let the pump click off the first time and check it that way, there can be different amounts of fuel in the tank due to incline at the pump, etc. The only way to truly check mileage in my opinion is to fill so you can see fuel in the neck so you know the tank is filled to the top the same way every time.

I've seen some people claim their truck gets 20+ miles per gallon, which may be true, but you have to ask how they checked it. If it was checked on level ground for 30 miles on the overhead, that's entirely different than checking it consistently over an entire tank of fuel with different driving conditions.

I check mine after every fillup and the overhead is usually around 1mpg less than what I'm actually getting.

 

[Rosco]

Spooled-Up is 100% correct. You NEED to fill the tank to the very top (fuel in filler neck) in order to hand calculate your actual mileage. If you just let the nozzle "click off" you can have a big difference from station to station. For one diesel fuel foams up as it is pumped into your tank. If one station pumps the fuel faster than another it may foam up more and cause the pump to shut off sooner than a slower pump.



FWIW my truck is usually within 1/2 a MPG.

 

[Casey Balvert]

You also need to average it out over several tankfulls. The ability to fill the tank the same each time varies with the speed of the pump and how level the truck is. Too many variables to draw a conclusion from one tank full. FWIW.



Casey

 

[tpuig]

I calculate my mileage, and it's not too far off the overhead. But I reset the overhead at each oil change. What's fun is to watch the overhead as you coast down a long hill. (safely, of course) I got to 99mpg indicated on the overhead on the I-5 Grapevine heading down into LA. Once I touched the throttle, though, it dropped back down to 18 or 19 where it usually lurks.

TP

 

[jsimpson]

Mine was spot-on before I put in the Bully Dog Torque Dog. Now it exagerates by about five miles per gallon. I guess the ECU is not the only computer fooled by the Torque Dog!

 

[XJSuperman]

Mine was off stock and now even more with the EZ. I avg 17. 5 MPG city and about 21 or so hwy... the overhead read as high as 36 MPG on the hwy at 70 MPG and as low as 6 MPG in the city. I don;t believe either number. But after almost 6,000 miles the mileage (hand calculated) keeps getting slightly better each tankful and I very rarely if ever look at the overhead. . it's crap in my book for mileage and I get a laugh out of folks who brag about their mileage when they're talking overhead and they should be computing it my hand.

 

[JHardwick]

You also need to average it out over several tankfulls. The ability to fill the tank the same each time varies with the speed of the pump and how level the truck is. Too many variables to draw a conclusion from one tank full. FWIW.



Casey

I prefer this method as a true indicator and I'm very lucky to see 17mpg with my driving habits ... ... ... ... most likely 15mpg.



I have seen way lower on a dead run from southern Ohio to northern Tenn loaded with 3 horses and living quarters stuffed with a weeks worth of camping ... ... ... . probably in the 10-12 range in all honesty.



But, I'm cool with that, considering that diesel is usually cheaper and a gasser with the same weight off the scales would never see 10mpg!

 

[GFritsch]

The overhead read about 2mpg under before the reflash, and now reads about 1 mpg over actual. I have found the speed kills economy. My last tank was 717 miles and 35. 4 gallons. It is possible to get over twenty if you are willing to drive in the slow lane at 60 with the truckers. I also use nuetral on downhills that will carry the truck. This seems to really help.



I would be very skeptical of any one averaging over 20mpg at over 70mph +



JMO

 

[rbattelle]

You also need to average it out over several tankfulls. The ability to fill the tank the same each time varies with the speed of the pump and how level the truck is. Too many variables to draw a conclusion from one tank full. FWIW.



Casey

Amen! Casey is a wise man. The only significant way to look at fuel mileage is to look at the average over many tankfuls... there's too many variables to look at just a couple tanks. My average fuel economy over 78 tankfulls is 18. 7 MPG. Attached is a chart of my fuel economy per tank. Highest ever was 20. 6 (10/5/03 - 10/12/03). Lowest ever was 16. 6 (first tankful... not meaningful, really).



-Ryan

 

[Briman]

62 tanks of fuel through truck since new. 33,217 miles/1,772. 45 gal fuel = 18. 74 mpg. Lowest milage/tank = 8. 67 (only took 15 gal to top off). Highest milage/tank = 21. 79 (40 mile per hour tail wind all the way accross Wyoming). Around 3000 miles with TT.



PJ, I don't think it is unusual to see over 20 with my truck, I certainly don't see it all the time. Oh yea, my over head is usually a fair indicator to what is really going on with mpg, not spot on but close enough for a quick perusal.

 

[John - K5AWO]

In addition to all the variables stated above, there is a minimum of one more: Temperature of the diesel in the tank plus the temperature of the diesel being pumped in. Warmer fuel will expand as most of us already know.



In the hot summers here in Phoenix, I notice a shortage in fuel economy as compared to winter time driving in cooler temperatures.



For me, it is extremely difficult to get accurate readings even though all of mine are hand calcs. The aux tank is computerized and I never know if ALL of the fuel in it has been consumed. The fuel level guage gives readings that fluctuate as much as 2 or 3 gallons depending on how the truck is positioned relative to level.



The digital guage makes the measurement errors all the more glaring. The analog guage in the dash is just about useless for meaningful measurements.



In addition to all else, the plastic fuel tanks are not rigid enough for accurate measurements on a single tank's usuage. Volume changes.



For my truck, the mpg's are somewhere south of 16 and north of 14 for typical city driving. One trip I made from Las Vegas to Phoenix netted something like 9. 5 mpg running at speeds of 95 GPS mph and RPMs over 3300 most of the time.



My two cents is that it takes at least a hundred gallons consumed to get close to fuel consumption measurements. Throw in different delivery pumps (if used) and it only gets worse.



I sure like the graph in the previous post. It is interesting to take the trends it shows more so than the actual measurement postings. I would suggest the large swings are a result of the truck being unlevel at the time of fueling up and probably a different delivery pump meter.



I would have liked to see altitude and temperature info in the same graph and gross weight pulled between fill ups. Ah, but how many of us would ever go to such lengths?



End of Book:

 

[rbattelle]

I'm generally convinced that the variations in economy I see are from changes in driving, different idle times, and (most of all) how much effort I put into completely topping off the tank. My whole purpose in all the data I take is to get good averages to watch for problems. Although I don't take altitude or temperature data, I do take location and date, so altitude could easily be determined and temperature could be estimated. Since I live in Ohio (flat as can be), altitudes are pretty much identical for all fillups. I generally get worse fuel mileage in winter because of all the extra idling waiting for the grid heaters to cycle. I don't use the fuel gauge to determine when to fill up... I use the trip mileage instead. That way if my fuel gauge breaks it really doesn't matter.



On edit: BTW, standard deviation for my data is 0. 75 MPG... that's pretty consistent, if you ask me!



-Ryan

 

[klenger]

rbattelle:



A question I have been asked a few times before: "What are you, some kind of an engineer or something?"

 

[rbattelle]

A question I have been asked a few times before: "What are you, some kind of an engineer or something?"

At least people are polite to you! When I tell people of my crazy mad-scientist schemes (or they get a look at my fuel log book) they generally roll their eyes and say "You're a dork".



To each his obsession.

 

[John - K5AWO]

I'm generally convinced that the variations in economy I see are from changes in driving, different idle times, and (most of all) how much effort I put into completely topping off the tank... ... ... ... .



On edit: BTW, standard deviation for my data is 0. 75 MPG... that's pretty consistent, if you ask me!



-Ryan

I have to agree with this statement. High idle times for me are usually in the summer with temps above 105 when my wife is along for the ride. She can't handle warm weather (why do we live here???) and so I turn on the electric fan and leave it idling to keep her comfort level where she wants it.



I would fair better if I did all my errands around town by myself (turn off the engine), but I enjoy a little company so I pay the penalty and let the engine idle while I run inside to take care of business.



I have never been disciplined enough to consistently keep your kind of records on a personal vehicle. Used to be forced to do it when operating company vehicles and always felt it to be an irritant, but did appreciate the good info it provided later when looking back.



Certainly agree that trending can provide clues to decreased performance and attention needed to maintenance.

 

[kmcdonough]

PJ,

My overhead was off by a couple of mpg, even after they reflashed it. I put 285 tires on my truck and had the dealer calibrate the seedo. Now my overhead is within 1 mpg. I don't get the mpg I see posted here though. I usually get 12-13 around town and 14-16 on the highway at 70-75 mph. My truck cap and AT tires have effected highway mileage somewhat. It seems like alot of guys with the higher highway mpg have the 6-speed transmission.

Kevin