Just how accurate is your overhead compared to Hand Calc?

[roperteacher]

Quick qestion, how are your HAND CALCULATED numbers comparing to the OVERHEAD read out?? I am seeing 20. 5 to 21. 2 MPG from the overhead computer (New was only at 18 MPG) Empty of course. Many are reporting that their Overhead is lower than hand calc?? My Hand calcs are right at 20. 0 for the most part, so my computer is very close. . how about yours? Reason I ask is in relation to the fuel fil up/useage problem I had this week. For some reason my truck's mileage dropped to 13MPG From Monday to Wednesday. Overhead never really changed, hovering at 21MPG the entire time. SOme replied to me and said that sometimes fuel is heated so it expands, fill up truck all the way, fuel cools and you lose volume, thus giving you the idea you actually burned more fuel? Just curious!

 

[XJSuperman]

My overhead is all over the place. I've seen as high as 36 MPG out of it and as low as 12. I've even reset it on long trips and it still jumps around.



My avg hand calc in town mileage is 17. 4 MPG. My best hwy mileage is 21. 6 MPG hand calc. I would not rely on the overhead for anything other than outside temps and the compass.

 

[akshadow]

On fairly long trips, say over 1000 miles my overhead is very near the same as calculated +-1mpg

 

[fritzimax]

+/- 1 Mpg... ... .....

 

[rbattelle]

My overhead is typically 1 MPG lower than acutal.



-Ryan

 

[RLighthall]

Since the reflash it's been pretty consistant at reading . 2 lower than hand calc...

 

[phloop]

Before I had the mythical mileage flash my overhead was always within . 25 of a gallon from hand calculation's. Know after getting the flash, the overhead is showing around a 1. 5 mpg more than hand calculation.

 

[DF5152]

very close but im sure that will change when i put the bigger tires on...

 

[JVolpe]

I have always filled up from 1/8 to full tank every time since new. I always reset the overhead mpg and hand calculate mileage.



Before the TSB at 8000 miles my overhead was always around . 7mpg lower than hand calculated. Since the TSB it is always around . 7mpg higher than hand calculated.



I dont really pay much attention to the overhead since it will have wide swings depending on driving conditions. My problem is the actual mileage that I get sucks!

Average 13. 5 city and 16. 5 highway, really expected more by now at 13000 miles.



Still better than the 10. 5mpg the Hemi got.

 

[KD7GYJ]

Overhead reads 17. 5, hand calculation is around 17. 8 MPG's, so they are very close. This is after the 18-037-04 TSB was completed. Mileage went up about 1 MPG, overhead increased about 3.



So the overhead and hand calcuated are very close.

 

[roperteacher]

That makes me feel better!! I'll just chalk off that one fill up as who knows what... . an error on somebody's part!!

 

[swexlin]

Before I had the mythical mileage flash my overhead was always within . 25 of a gallon from hand calculation's. Know after getting the flash, the overhead is showing around a 1. 5 mpg more than hand calculation

Exactly the same for me. Before the flash, was about 0. 2 high - now reads anywhere from 1-1. 5 high post-flash. And I almost always fill up at the same pump, to the brim, so I am getting consistent data.



Actual mileage has only improved slightly, but we are in winter, and on winter fuel, so I won't really know until spring whether or not the reflash helped mileage.

 

[KD7GYJ]

Reason I ask is in relation to the fuel fil up/useage problem I had this week. For some reason my truck's mileage dropped to 13MPG From Monday to Wednesday.

Are you sure someone hasn't been slurping your tanks elixer??

 

[RKerner]

Before and after...

Exactly the same for me. Before the flash, was about 0. 2 high - now reads anywhere from 1-1. 5 high post-flash. And I almost always fill up at the same pump, to the brim, so I am getting consistent data.



Actual mileage has only improved slightly, but we are in winter, and on winter fuel, so I won't really know until spring whether or not the reflash helped mileage.

Before TSB 18-007-04, the overhead read 5. 5% below actual mileage. After the TSB, the overhead reads 11. 3% greater than actual mileage. 13. 5K on the ticker, 3K after the TSB.

 

[joeyou]

O/H Calc Diff

17. 3 18. 13 -0. 83

17. 5 18. 19 -0. 69

19. 2* 18. 48 0. 72

19. 2 18. 93 0. 27

19. 6 18. 63 0. 97

18. 8 17. 29 1. 51

18. 5 17. 76 0. 74

19. 3 18. 66 0. 64

17. 2 16. 87 0. 29

17. 9 17. 21 0. 69

19. 1 19. 10 0. 00

18. 2 18. 25 -0. 05

18. 7 18. 54 0. 16

17. 7 17. 31 0. 39

16. 2 16. 43 -0. 23



*after reflash

 

[KennethS]

My overhead shows 15. 3 and I check It and was 18. 4. Anyway I am going to keep a close watch on the overhead . I don't trust the darn contrapshion or whatever what you want to call It. Have not had my truck out of the garaage for 3 weeks. Weather bad and to hard to clean up by myself.

 

[blackholehunter]

Mine must be the only one that the o/h reads higher than actual. I don't write my numbers down but I always check at the pump and my hand calc have always been . 8 to 1. 5 mpg less than the o/h. For instance I filled up today and o/h said 17. 1 and hand calc was 15. 5. My truck only has 4k on it but I hope mpg gets better as it is broken in. My 93 and my 1996 both 2wd which I realize weigh less and have less resistance both got 18. 5-19. 5 doing the same driving that I am doing now.

 

[bpenrod]

JVolpe,

Did you get the speedo corrected for the larger tire? If so do you know what the put in for tire rpm's and if it was the right number. That would throw off the mileage calc's.



By the way, my overhead is a consistent 3 to 3. 5 mpg higer than hand calc but I attribute that to the fueling boxes. Prior to that is was always within 1 mpg if I remember correctly.

 

[JVolpe]

bpenrod,



I did have the dealer service department reprogram the "Pinion Factor" for my 315s. I had them use 605 RPM(Revolutions Per Mile). The BFG site documentation shows 601 for RPM. I used 605 because some tire calculators come up with 605 and I have spoken with another Ram 2500 owner with the same setup as mine. He has verified 605 on his truck with GPS. I have verified mine with known distances multiple times and have been +/- . 1 mile off over 100 mile distance.



The overhead is just subject to too many different factors to be exact. It is just a general indicator of how driving conditions are affecting the mileage. I reset the overhead on every fillup. It is consistantly . 7 mpg over hand calculated by the next fillup, but in between it jumps all over depending on city/highway/traffic/speed/etc.



Thats my opinion anyway, and you know about opinions.

 

[bpenrod]

I know what you mean about the rpm, that is why I asked. It seems like the calculations never match the number provided by the tire company. I have larger rims/tires on order and was wondering if I should go with the calculated number or the manufacturer provided. Guess I'll try the calc and use my GPS to verify it and then try the other.