My experience with the trip computer is limited as my vehicle is brand new. The two times that I have hand calculated my fuel mileage, the computer has been exact. For example I hand calculated once that my mileage was 21. 29. The fuel computer showed my mpg as 21. 3. A couple of days ago I again hand calculated my mileage and came up with 18. 81. My computer showed 18. 9. Like I said my experience is limited due to the vehicles newness. My vehicle is stock and according to some other members, bombing really throws the computer off.
My instant mpg will shoot up to 99mpg when I am coasting in gear. I suspect that it stops at 99 because two didgits are as high as the computer will go. I can't say for sure, but I expect that the engine uses none, zero, zilch fuel while coasting in gear. I have a Volkswagen Diesel (TDI) and have verified that it uses no fuel whatsoever when coasting in gear. A friend of mine hooked up his laptop computer with some special software to my VW as I was driving. It showed various fuel parameters and showed that the car was using no fuel while coasting in gear. I didn't believe him when he initially told me this but seeing is believing. Like I said, I SUSPECT that the Dodge Cummins is the same way, however, I may be wrong on this.
I find it hard to keep the fuel computer instant mpg the same even using the cruise control. The slightest incline (or decline) will affect the readout as well wind speed and direction. For now though, I am extremely happy with the computer. However, time will tell.
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