[BorgHunter]

This is a trick more than one automaker does in the interests of creating a "green" (or "eco") trim, and usually they do it by speccing tires that are all-in on prioritizing fuel efficiency, sometimes with a higher recommended tire pressure too. The tradeoff is that they sacrifice some traction, and either have to go with smaller wheels (big wheels = luxury = $$$, for some reason--Toyota does this with the Camry Hybrid, putting 16" steelies on the cheap LE trim but 18" or 19" alloys on the others) to keep the same ride quality, or just accept a stiffer ride.

It's worth noting, though, that as fuel efficiency goes up each increment in mpg is less valuable, so 6-7 mpg on a car that already gets 50 mpg is not as useful as 6-7 mpg on a car that gets 20 mpg. This is because the thing you're trying to minimize (fuel burned) is in the denominator of the measure. Countries that measure fuel economy in L/100km don't have this problem. Engineering Explained does a good job of showing how the math works out: https://www.youtube.com/watch?v=oLQmwOX6Xds

[gnicholas]

They do tire tricks, but here it was actually a different battery chemistry that made the biggest difference.

I completely agree with your assessment of the MPG difference. I tried explaining this to my relative, to no avail. The number for one vehicle was under 50 MPG and the number for the other vehicle was over 50 MPG, so it was a mental sticking point that I could not overcome.