MPG question

When going slowly enough with the engine running you are "wasting" some fuel (because you are not using enough power to overcome the waste that is always there) so mileage drops with speed below about 35. Prius (and many other cars) gets its' best mileage at around 35 to 45 MPH, and maybe even a bit higher. Above about 55 MPH air drag becomes high enough to start to drop mileage. Air drag increases faster than speed so it really begins to dominate above 65 MPH. This is why Prius gets better city mileage than highway mileage.
But, sometimes all this gets messed up. Which is why YMMV.

 

One of the ways that Toyota gets good mileage is that they put in a very low horsepower engine.
Most engines are wildly over sized, and spend a great deal of their cruising speeds at almost idle, with the throttle plate only slightly cracked open. The cylinders are trying to pull in air but are unable to because the throttle is almost closed, this is called 'pumping loss' as it is an inefficient way to pump air. (You can also get high speed pumping losses when the intake manifold is not large enough to flow all the air the cylinders attempt to pull in)

By having a lower HorsePower engine, the throttle stay open more often and cruising is not near idle. Obviously, if you go slow enough, even the wimpy Prius engine starts having pumping loss. Under those situations, the computers may use EV to avoid light loads, alternately it may stay in a gas mode, but charge the battery to provide enough demand to avoid pumping loss. If the battery is already charged, EV is the choice.

I find this to be below 37 MPH in my v.