Understanding Trip MPG vs. Calculated Trip MPG

[Larry Riley Jr]

Sorry for the long post, but hopefully this is some interesting food for thought:

 

Like many folks of this forum, I'm currently keeping a very detailed log of the current tank of gas. I reset the Trip 1 screen when I last filled up, and after every trip (no matter how quick), I record the elapsed time, total trip distance, MPG, fuel used, EV miles, DTE, and trip MPG (the one that shows up after you turn off the ignition). From here I've built a spreadsheet that computes all sorts of data points that is really helping me monitor how well my driving habits are improving in the car, as well as understanding the impact of each trip on the RAFE, the actual MPG of the ICE, how much gas I actually have left in the tank, etc.

 

One of the things I've noticed, as has been noted elsewhere on this site, is that there is usually a discrepancy between the reported MPG for a trip, and what it calculates out to when you divide the distance travelled vs. the fuel used as displayed on the screen. This is especially true on short trips. For example, according to the trip computer, I drove 2.0 miles earlier tonight on the way back home from an errand, and burned .04 gallons, which calculates out to 50.0 MPG. However, the reported trip MPG was 46.5. Although they are different, both numbers are actually right . . . sort of. It's all about the rounding, which I assume is what is happening here: the distance driven could actually be anywhere from 1.95 to 2.04 miles, and it would display as 2.0. The gas burned could be anywhere from .035 to .044, and it would display as .04. So if I actually drove the shortest distance that would show 2.0, which is 1.95, and I burned the most gas possible to show .04, which is .044, then the worst possible mileage that the car could report to me is 1.95/.044=44.32 MPG. Conversely, using the furthest distance and least fuel, the best possible mileage is 2.04/.035=58.29 MPG. So in other words, it's possible for the car to display 2.0 miles driven and .04 gallons burned, yet display a mileage anywhere between 44.3 and 58.3 when I turn off the ignition! So as long as both the reported trip MPG and the calculated MPG fall within this range, then there really isn't a discrepancy; it's just that we'll never have the actual numbers past the decimal places displayed on the screen, even though I'm sure the car has those numbers out to the nth place internally and uses them in it's calculation.

[hybridbear]

Great explanation! I also am impressed by your detailed record keeping on trips in the car. I'm sure it's helping you improve your hybrid driving skills.