Dodge Repair: mileage display, powertrain control module, transmission control module

Question
My Voyager has an overhead display that shows miles per gallon and other stats also. The miles per gallon is now off by about 25% compared to when I calculate it myself. How does this system work? Is the input off of the manifold or what and can it be fixed to be accurate again?

Thanks,

Answer
Black electrical tape.

Honestly, this and the advertised MPG on the New Car sticker are probably the two most unfixed customer concerns out there.

This overhead module has 6 wires on it.  Two go to an ambient temp sensor located behind your front grill area, one is ignition power, one is ground and the other two are tied in to what's called the Bus network.  This network includes the transmission control module, powertrain control module, body control module and things out of the picture like radio and airbag module.

Bottom line is that since it's conencted on the bus there's no telling where it gathers all of it's information from but most of it has to come from the body control module and the engine control module.

The thing I try to tell people about this indicator and every other one on the car is that they are not very accurate gauges.  Crunching the numbers like you've been doing is the best way to find out mileage.

You may want to verify that your tire pressure is set properly.  Since the overhead module is probably using a predetermined graph of what mileage should be at this RPM, at this speed, at factory tire pressure, and this ambient temperature then you should try and keep the one thing you can control set properly.

Additionally, making sure you're running the same size wheel and tire as factory may make a difference as well.  Finally, the transmission computer can be told that it has different size tires on it.  This is done for speedometer/odometer reasons.  Likewise I'm sure this can affect your expected MPG versus actual.

This was a great feature and I'm sure it may change someone's perspective the next time they want to romp down on it but honestly, it just isn't accurate enough and it leaves you with a guage that's almost useless.
Doug