MPG vs GP100 - a numerical (conceptual) for the average automobile owner. Here's why ...

[sear]

According to this article both MPG & GP100 values are included in the EPA fuel economy ratings listed on the window sticker of new cars.
MPG is the more familiar value. And it does provide a useful approximation for comparison between a Toyota Prius and a Cadillac Escalade.

"... the gallons/100 miles rating makes it easier to compare the efficiency of different cars and estimate their likely annual fuel cost. ...
The advantage of measuring fuel consumption this way is that it makes comparisons easier as fuel efficiency improves for a specific vehicle. That’s because the differences are linear. With miles per gallon, efficiency is graded on a curve. For example, for a 15-mpg car, a 5-mpg improvement is a 33-percent gain. But that same 5-mpg upgrade for a 30-mpg car is only a 17.5-percent improvement to a vehicle that is already using half as much gas.

https://getpocket.com/explore/item/...g-a-car-s-efficiency?utm_source=pocket-newtab

bold emphasis sear's

This source says both values are on the window sticker.