i keep hearing the overhead mileage meter is off, how bad are they off?

From what I have read here and my wife's Dakota they vary. I found if I reset the Dakota after every fill up the end result was fairly close. Hers is all stock. If you change tire size that will throw them off. If you want really accurate milage like to the .10 or even the 1.0, the overhead just aint that close.

All things are realitive... Tire error... my factory option tires read dead nuts on mph compared to my GPS. I am still running those tires & my speedometer is right on. Any changes in tire diameter (reguardless of size) will affect the actual miles run although the hand calulations using the factory instruments may read he same. When I was running stock engine, my overhead agreed exactly with my hand calculated MPG. With my XZT+ chip, the overhead MPGs are off varying with what program I am running, but the +35hp is reading about 1 mpg more than hand calulated. Still differences in driving or load that pushes the mpg up or down are still valid though the actual numbers are off. If the OEM maker made them adjustable, that would just encourage owners hot rodding their trucks. (If they didn't wreck the power when they smogged the trucks, we wouldn't need to hot rod the trucks... tell it to the EPA!) Sure would be nice if someone figured out how to modify the overhead to be adjustable.Some of the scan tools have adjustable parameters & therefore can give correct readouts for tires changes & different power tweaks.

Some of the scan tools have adjustable parameters & therefore can give correct readouts for tires changes & different power tweaks.

Russ is right... Like I'm running a ScanGauge II and dead on... I'm capable of calibrating for tire size and fueling changes where the overhead computer is off once you change tires or fueling mods...

http-~~-//www.youtube.com/watch?v=SRQdoPs_tLo

It used to be dead on, but after I added a trans flow fuel tank it is not accurate anymore, don't know if they are really related to each other. Has anyone else had this issue?

It used to be dead on, but after I added a trans flow fuel tank it is not accurate anymore, don't know if they are really related to each other. Has anyone else had this issue?

My truck when stock read on the lie-o-meter as 2.2 mpg higher than hand calculated. Since then I increased the tires and used my Smarty and my GPS to correct the change. I changed out to a larger tank. And my overhead is now 2.1 mpg higher than hand calculations. It's never been accurate no matter what I did. But I can use it to see if my mpg is increasing or decreasing while on my trips.

I track the error every fillup, the error is never the same 2 tanks in a row and for my truck varies from 8% - 35%. Here is small sample.

I guess I have been wasting my time trying to see if the overhead is accurate, I had checked it for 12,000 miles on my Alaska trip, and every fill up, I thought it was me.

Russ is right... Like I'm running a ScanGauge II and dead on... I'm capable of calibrating for tire size and fueling changes where the overhead computer is off once you change tires or fueling mods...

Great, I just signed up on this forum, and ALREADY spending money

Great, I just signed up on this forum, and ALREADY spending money

Hang in there, you're only at the starting line ....... Much more to come ..........

Mine seems to be on avg. about 6mpg proud of hand calculated.