Smaller wheels highway question?
 

People claim that bigger and heavier wheels help with highway gas mileage because of inertia

If that’s true, then why are all of the epa highway numbers better with trims that have smaller wheels? Wouldn’t just the city mpg be better and the highway mpg be worse?

So are bigger wheels better or worse for highway mpg?

Inertia does not help on the highway, unless you're trying to pulse and glide in a manual vehicle, and being able to lengthen your pulse and glide cycle makes you more likely to actually do it.

I can't say exactly why larger wheels hurt on the highway, but I have a few ideas:
-Wheel surface is generally less aerodynamic than tire surface
-Larger wheels are heavier
-Tires on upper trim vehicles may prioritize grip or noise, rather than economy
-Tires on larger wheels may be wider

It doesn't relate to your question exactly, but a larger tire diameter, all else equal, should reduce rolling resistance, because the deflection is lower. A larger diameter tire can be narrower and achieve the same contact patch, which further helps with aero. All else equal it should be heavier, however, and lift the vehicle firther from the ground.

The problem starts with "People claim...".

Reductio ad absurdum -- Monster trucks would get the best highway mileage.

Big could mean tall and narrow or short and wide. Weight could depend on carcass construction (# of plys), thus is dependent on vehicle weight..

It's final drive gearing.
For example I replaced the clown tires on the Dodge Dakota wich were 215/70r16 with proper 30x9.5r16, it lowered my highway RPMs by over 200rpm at any given speed. It gets about 1mpg better.
OEMs can run clown car tires because they run numerically lower final drive gear. So never expect to go smaller and get better fuel economy it's a trap.

I was going to downsize the new Prius oem 17 inch wheels to 16 inch wheels that are lighter weight

Big wheels with rubber band tires vs little wheels with flotation tires.? For equal weight, high vs low internal pressure?

I read this question as asking about wheel diameter with a fixed outer tire circumference. For example, in the 5th gen Prius, you can get them with either 17" or 19" wheels, but both come fitted with tires that result in virtually the same rolling circumference.

Exactly. The 16 inch wheels also have the same circumference. Just more rubber for a better ride and less chance of wheel damage. The overall weight for the 16 inch set up will be less than the 17 inch one

Tesla shows in their ranges about 20 miles of range added when downsizing

And Prius 19 inch highway wheels go from 50 mpg highway up to 54 highway when dropping down to 17 inch wheels

Stuff that works on electric cars don't work on gas powered ones.
Gearing and motor rpm isn't as important on an electric motor.

It's BMEP vs back-EMF.