Going downhill at 70-80 mph

With a Scangauge II or a similar device in your OCD-II port you can see the actual (alleged) engine-load in real time. Personally, I never hit 70 mph. It's a Prius, people are not going to like you anyway.

 

No engine uses has when doing engine breaking. No modern ones, at least. Old carb engines did use

So, don't worry about it

 

I think the engine will use some fuel during the startup cycle, it won't be just doing engine braking in this phase as it needs to get up to temperature (to heat up the cat, etc.)

 

If it's using any gas at all in that scenario, it's miniscule. I think the only way to save more is with a pure BEV or maybe a Prime.

 

The ICE has to turn (for power OR braking) when above 65 M/H because otherwise the MG would over-rev.

 

Not necessarily. If the engine is running downhill while braking the engine alone will be using so little fuel that it itself will be exceeding 99.9 MPG.

 

I'm not sure it actually has a fuel flow meter? They are a relatively expensive component, so trip computers are usually calculating based on airflow and other data - hence they're not the most accurate things around.

 

I've read in other places that that's how it works. Ut's just inferred from other data, as you said.

 

Keep in mind that there is no legal 75 or 80 mph anywhere in Washington, 70 is the highest legal limit for cars. In my corner of the state, there are always some cars staying legal.

And 60 is the highest legal limit for trucks. And a few trucks even obey it, even on downhills. And more will keep down to your Pip's MG1-imposed ICE-off speed limit. Can you find and stay behind some of them? Many drivers upset with slow isolated Prii, suddenly switch lanes and try to beat them around the slow truck just ahead. This is often a convenient way to go slower and shake those folks who think you should go faster.