Thanks Ethan... You're absolutely correct.
The point I was trying to add is that MPG can come right down to the driver's habits as well.
Comparing 2 different models or even 2 cars of the same make brings new variables to the equation, as you have mentioned.
Comparing only the driver, I get 5 MPG more than when the wife is driving.
Here, the tires, location, altitude, barometric pressure, type of road, fuel, tires and tire pressure, gross weight, moon's phase... all the variables are the same.
The only variable that changes in my example is who's driving the car.
The wife loves to constantly adjust her throttle. It's very slight, but I notice it every time she drives.
I'm more of a throttle featherer. (is that even a word? )
My point is that when comparing one person's model to another's same make/model... driver habit can and will make a big difference in MPG ratings, even with the slightest difference in driving habits.