OHC vs hand calc discrepancy

[Wardog]

I just got back from a 3K mile trip pulling a 10K fiver. The OHC was off significantly compared to hand calculations. The OHC would read 13 and the math would show 11. 3. Sometimes the difference was as high a 2 mpg. I have had the TSB for smoke and mileage done. Lately my mpg have been going down even though they're supposed to improve with more mileage (I've got 17K on her).

Anyhow, I'm going to call the dealer and see what they have to say. Anybody have similar problems?

 

[Okey0]

i made a 2500 mile trip in june pulling a boat plus slide in in the back. went from wyo to iowa. the overhead read 11. 2 when I got to iowa. i reset it for the return trip, and when i pulled up to the house it was at 11. 2 again. the next day i added up all the fuel i used, and it averaged out to 11. 2. I was surprised that it turned out so accurate.

 

[Tim]

Mine has never been accurate. I hope it's accurate now... . it says 18. 2 on this tank and the last one was 17. 9, and rising.

 

[DPKetchum]

Its a AVERAGE which at times the actual mileage may drop quickly lower. It works as designed.

 

[WCPoe]

Overhead -vs- hand calculated

My overhead was always within about two or three tenths of my hand calc until I had the reflash for the smoke and improved mpg. Now its always off by one or two mpg's. I wish I never had the reflash done. Just my 2 cents worth.

 

[thejeepdude]

Its a AVERAGE which at times the actual mileage may drop quickly lower. It works as designed.

Yeah it works as designed... POORLY! I have run a full tank on flat ground 100% highway with no stops. My OHC was still over 2MPG optimistic. It said 21. something and my real hand calc'd mileage was in the low 19's. The OHC is only good for relative comparisons, not accurate measurment, at least on my truck.

 

[Buffalo]

Steve in AZ.



At 4:30 PM, yesterday, mine was at 117 deg's. , at Camelback Rd. & 7th Ave. Later on, driving north on 7AV, it dropped to 115 deg's. That area is a "greenbelt" with a lot of lawns & trees. Keeps it cooler.

The radio, at the same time said it was 115 deg's.

Any number of times the radio weather reports & the trucks thermometer have "agreed" on the temps. Leads me to believe that the thermometer on my truck is pretty accurate.



Not too sure about the fuel mileage, though.



Joe F. (Buffalo)

 

[BrianJ]

My overhead was always within about two or three tenths of my hand calc until I had the reflash for the smoke and improved mpg. Now its always off by one or two mpg's.

Same thing happened to me when I had the reflash. I was always within two or three tenths +- before the flash, post flash the computer is always 1. 5-2 MPG over hand calc. Before and after the flash I would fill to the neck, so I know the hand calcs were accurate. That takes too much time, so now I just add two gallons past the "click" of the pump, which should still be fairly consistent for mileage calcs. There wasn't a noticable mileage change after the flash. Before and after I can get high teens - low twenties if I stay off the 'go' pedal and stay 70 or below on the highway.

 

[DCornelius]

Same thing happened to me when I had the reflash. I was always within two or three tenths +- before the flash, post flash the computer is always 1. 5-2 MPG over hand calc. Before and after the flash I would fill to the neck, so I know the hand calcs were accurate. That takes too much time, so now I just add two gallons past the "click" of the pump, which should still be fairly consistent for mileage calcs. There wasn't a noticable mileage change after the flash. Before and after I can get high teens - low twenties if I stay off the 'go' pedal and stay 70 or below on the highway.

Mark me down for the same exact problem. I was within a couple of tenths before the flash, and now usually 1. 9 to 2 mpg off after the flash. I too wish I hadn't had the flash done, because I've seen no appreciable difference in mileage or anything else. I was actually impressed with the overhead calc before I had the flash done. I couldn't believe it was as close as it was.



I've waited months to post anything on this, and figured that everyone was tired of "flash" talk, but I saw this thread and had to post my experience.



Thanks,

David

 

[Tim]

Yeah it works as designed... POORLY! I have run a full tank on flat ground 100% highway with no stops. My OHC was still over 2MPG optimistic. It said 21. something and my real hand calc'd mileage was in the low 19's. The OHC is only good for relative comparisons, not accurate measurment, at least on my truck.

You have an NV 5600 I take it?



On the temperature issue... .



Mine always reads a few degrees hot. It said it was 99 one day, and in reality the temperature never broke 92.

 

[thejeepdude]

You have an NV 5600 I take it?



On the temperature issue... .



Mine always reads a few degrees hot. It said it was 99 one day, and in reality the temperature never broke 92.

Nope, 48RE. Also, I've noticed the temp as being within a degree or so on my truck. Well hey, at least one function WORKS.

 

[10-9jc]

OH MIleage usually about 1 mpg off - better than it really is.



Temp trivia: "Back When" wife had an '85 Park Ave. (now long

gone) - anyway had a temp indicator that was always -exactly-

right on! Even checked against thermometer at home.



So why can't D/C get it right 20 years later on (my) 05 Dodge?

Don't know where sensor is - but can be 10 degrees - or more - off.

 

[RMachida]

Don't know where sensor is - but can be 10 degrees - or more - off.

Manual says the temp sensor is below and next to the drivers side headlight behind the grill shell...



but I haven't looked.

 

[keimmmo]

You guys are griping about your computer being off only 1-2 mpg. The computer on my 01. 5 has always been off 2-3 mpg... at least I get 2-3 mpg BETTER than it shows!



Steve

 

[fkovalski]

... I had the reflash for the ... improved mpg. Now its always off by one or two mpg's.

I have less than 1,000 miles since I got the '05. I am still monitoring via written log compared to the overhead.

... I wish I never had the reflash done.

Don't want to hijack, but any opinions as to whether or not to look into getting the reflash done? It does smoke a lot more than my '01, but if it deters MPG then I will hold off on this.

 

[mdepot]

Does the OHC take into account your axle ratio and tire size? If your speedometer/odometer is not accurate, then I would think the OHC calculation would be wrong also. I wonder if when they are reflashing if they are overwriting values like the axle ratio and tire size back to some default so then it starts calculating wrong?



If it turns out that computer settings like the tire size are getting reset, might that also affect proper ABS activation? It's just a guess, but it may be worth looking into. A good test would be to run a measured mile (or use a gps) to see if your speedometer and odometer are still accurate after the reflash.

 

[BrianJ]

A good test would be to run a measured mile (or use a gps) to see if your speedometer and odometer are still accurate after the reflash.

I did that with a GPS after the flash and the speedometer and odometer were very close to the GPS readings. I also had the dealer check the pinon factor recently and it was right on.