Bo, I dont know what algorithm the Dodge mpg calculator uses, but here is a typical method.
- ECM stores mileage average data for the last X number of long miles (could be as much as 500-800 miles).
- ECM also calculates mileage for the last X number of short miles, which might be as small as 10-50 miles.
- displayed mileage is an average of the "long" miles aganst the "short" miles, with the "long" miles about 80% weighted.
- the algorithm is adjusted for different types of cars/trucks based on typical useage and a lot of feedback from customers and engineers in the field. The same engine and trans package, in two different cars, may display differently due to typical age group of the buyers.
The purpose is to give the driver a more realistic view based on his overall drivng condition and habits. If you tried to do a shorter running average, for example comng down Floyd Hill into Denver on I70, you'd look at your latest average and say, "gee, I dont need to stop for gas...I'll just head on out into Kansas with my 1/4 tank!"
Even the short term MPG selection is somewhat a blended number. Actual instantaneous mileage is changing so rapidly, we had to store the readings as long data files and then average particular sections to see what was happening on somethng like...say... the top two miles of Baker Grade where oil temps are up in the stratosphere, trans temp is so high that the torque converter clutch has been commanded into lockup by the ECM, and throttle control has taken you down to 28 mph at wide open!
It sounds goofy, but it is a remarkably good method to give the driver a reqlistic view of his/her driving habits.
By the way, for those interested, calculating mileage the old way (miles driven divided by gallons) will not work on certain hybrids. This is because of several factors.
- fuel tanks are pretty small, and so a 0.5 gallon error is a big percentage miscalculation
- charcoal canister capacity is large, and you can force overfill into a flooded canister condition..the car will purge/clear that canister as quickly as it can...but its not helping the mileage as much because it is a duty cycled vacuum draw and not injected at the intake ports. That overfill fuel makes your last mpg calculation look really bad!
- fuel tank shell contains a semi-collapseable bladder that is used to keep air space off the gasoline, for reduction of evaporative emissions. If you get really aggressive with the gas pump, you can overfill into the shell (gallons!) The computers will work like crazy to get that sucked out through the EVAP system. The customer, meanwhile, is convinced his Prius only gets 15 MPG.
- bladder expansion changes from fill-up to fill-up depending on barometric pressure, fuel temperature, kick-off pressure of individual gas station pumps, angle of the gas station ramp, condition of the nozzle insertion tube (dented or badly scratched means less fill-up compared to another day and place).
Thats why so many magazine mileage tests of first and second generation Prius were wrong.
The tip here, for all of us, is DO NOT top off ANY modern vehicle. It forces the excess fuel into the EVAP charcoal canister and the computer gets right to work cleaning it out. I used to occasionally speak at New Owner Event nights, and had a favorite story to explain this.
"When you overfill your tank, the emission system can become flooded. Your car will take care of itself, but it doesnt help your gas mileage. The next time you feel like topping off your tank, go to the gas station with 4 one-dollar bills in your pocket. When the gas nozzle clicks off by itself, hang it up and take the 4 one-dollar bills out of your pocket. Put 2 bills back in your pocket, and then drop 2 on the ground and drive away. You were about to waste that 4 dollars....now you have saved yourself two of them, and made somebody else's day!"
Brave new world, aint it?!