ICE driving style (high or low RPM)

... As in, don't put a manual transmission in a too-high gear, thus running RPM too low?

There is no user gear selection to be done in a Prius, its HSD system handles all that automatically. You can't under-rev it. For any given power output level, the ECUs mostly pick the engine torque and RPM from a pre-determined load line, then adjust MG1 and MG2 to provide the needed 'transmission' ratio.

When you find the engine RPM low, that is because it isn't needing to produce much power in the first place, not because it is running at a too-low-RPM-too-high-torque condition as you might do in a manual transmission car.

 

Do some reading on Pulse & Glide technique... In general, when accelerating, the ICE is most efficient at roughly 2/3rds throttle... So relatively hard acceleration to get up to speed is more fuel efficient... Then the Glide comes in, which uses almost no gas.

Also I've read on here that when climbing mountains on a highway, 3000rpm is most efficient. But haven't got RPM data set up yet to study if that's true yet.

 

Yeah if you want to get the revs up, just go up a hill. That happens every time we go to our dentist: there's a steep hill climb, maybe a kilometer. Fair amount of twists too, so varying rev's as you take the corners. Good workout I think.

I figure just drive conservatively. Stick to the speed limits or under, keep a good buffer in front, coast up to slowdowns as much as possible. With the Hybrid Synergy Indicator (bar display), aim for 75% throttle when accelerating, when possible/practical.

 

I’d say generally keep the RPM low, but occasionally floorboard it to keep the engine in top shape.

 

Here are the Prius 1.5L (Gen2) and 1.8L (Gen3) BSFC (brake specific fuel consumption) charts:



The shaded areas at the most efficient. Prii operations are programmed to stay on that solid line passing through that shaded area.

Compared to older non-Atkinson engines, these engines have a very broad plateau of almost-peak efficiency, so you can operate over a very broad range of RPMs or loads and still be 'close enough' to peak efficiency. I.e. you can 'go with the traffic flow' and still be within a hair's breadth of best efficiency.

On a number of steep hills that I travel, 3000 RPM is simply not high enough to keep up with prevailing traffic flow. Don't cause a big traffic backup behind you just to eek out a small efficiency improvement. Traffic law requires slow vehicles to take turnouts to let the other cars pass, and these extra stops and subsequent accelerations back to travel speed will waste most or all the saved fuel.

In a Prius at highway speed, Pulse & Glide doesn't save anywhere near as much fuel, or even any at all, compared to the same techniques used on old style Otto-cycle engine cars. Those old cars had a lot to gain from P&G, but most of the possible savings have already be captured and built in to the Prius via other means.

Pulse & Glide does work well on a Prius driven below maximum auto-ICE-Off speed, about 42 mph in a Gen2, 46 mpg in a Gen3. The savings come mostly from letting the idling engine completely stop spinning during the Glide phase. But above those speeds, the engine must always spin to protect MG1 from overspeed, and there is almost nothing else left to save. Especially so in a Gen3.