Flumoxxed or Bamboozled ? : On board MPG calc.

06Fargo wrote:
Our Dodge with 5.9 diesel is pretty accurate.

Most folks on various Dodge diesel forums call their overhead "the lie-o-mater" LOL

Mine is consistently inaccurate, so I guess it's sorta accurate in an inconsistent way :B

I use an app called Gas Cubby to track fuel use, mileage, and scheduled maintenance. Makes things simple, keeps all the data in one place.

Our MH has a SilverLeaf computer in the dash. It computes and displays a rolling mpg, an instantaneous mpg and and average mpg for a trip. I find the rolling and instantaneous mpg's to be pretty useless particularly if the terrain is varying, but I zero the trip settings everyday so I can compute the average for the day.

I find that on a RELATIVE basis the computed daily average is highly reliable (it excludes fuel used by the generator and during periods of prolonged idling with the transmission in neutral); I can clearly see the effects of crosswinds and headwinds and of climbing mountains, etc. With a big fuel tank I buy fuel on a rather erratic basis, so the daily computed mpg's provide me with a more regular statistical basis for analyzing engine performance.

06Fargo wrote:
Our Dodge with 5.9 diesel is pretty accurate.

Our 04.5 Dodge with 5.9 diesel is no place close.20% +/- off.
I wish I could really get the MPG that the OB readout claims>

Our Dodge with 5.9 diesel is pretty accurate.

However it does it, the one in my Ford truck is pretty much right on. When pulling in for fuel I always check the computer for the fuel used value. Then I run the dispenser at full boar until the gallons get to within 1/2 gallon of the fuel used value. Run it slow and the final total pumped is within .1 or .2 gallon. Try it the next time, you'll be surprised.

Not sure about cars, but my Workhorse chassis measures the fuel being used, and calculates an instantaneous MPG value or an average value - the average being a rolling average over the last 50 miles driven. Instantaneous value is pretty much useless except for training your right foot to be light. I leave mine set on average and it shows a reasonably accurate reading that changes with the terrain and driving style, but gives a pretty accurate reading so you can plan fuel stops, etc.

"Sample" was a poor word choice on my part. As far as I know it is a continuous read of fuel delivery and distance traveled.

I don't think it analyzes anything. It reads fuel flow and road speed and does the math. Maybe it is sampling, as Doug said, rather than counting every bit of data. They always seem optimistic, though!

When you complain to the dealer about your mileage use what you calculate.


When you are trying to sell your vehicle use the On-board mileage calculator :)

Hand calculate, My MPG indicator on the dash sometimes looks right when going a steady speed, say 60 MPH, but I have seen readings of 35 MPH going down hill with no throttle, so I can brag about that but know better that it is just a instant calculation made by the computer for that time period and when I start back up that hill with a low gear and full throttle it will fall way back down. So the computer will deal with averages, hand calculate over several tank fulls and you will get pretty close to your actual MPG.