ICE vs EV - Driving around 35 mph

I'm in a similar position regarding ownership and city driving, and I continue to experiment as you have. I'm steadily coming to the conclusion that, as in driving any ICE-only vehicle, steady driving with a light foot is the best way to go. For our conditions (30 mph limit mostly) this results in running down EV with SoC dropping to 40%, then ICE to 45%, and repeat. I throw in some pulse-and-glide but I don't fixate on it.
It's summer here, about 75F in the afternoons. A city route I have taken a couple of times since Christmas is about optimum - 20-25 miles, half flat, one long shallow hill, then gentle rolling; very little elevation change. Weather was fine, road smooth and dry, very light traffic, tires/tyres at +8psi. I'm into anticipation and avoiding loss of momentum.
For both these trips (no stops apart from a couple of reds, from home back to home), I got 84mpg indicated, my best yet.
Other routes, varying traffic, more stopping, means more gas, but over the summer on these roads I haven't done worse than 67mpg. If I add some city highway (motorway) into the mix ( I sit on 50-55) I still do no worse than 62. On the highway I use cruise if the traffic is light.
This might sound like Aunt Daisy driving, but cars are smaller here and speed limits slower (national limit 62 and enforced). In 30mph limits I just drive with the traffic. On the highway there are lots of faster drivers, but others as slow or slower, and I've only had a couple of tail-gaters in 16 months ownership, who shifted when I slowed on them. My C is an ideal car in these conditions.

So, for what its worth, I vote for letting the car do its thing, even if that is lots of EV driving.

 

Best way fo save fuel: keep EV to glide or very low demand.
To accelerate, use ICE (step harder on gas).
I believe the operating manual refers that.

 

The expert hypermilers use both strategies, and also other techniques. Depending on terrain, traffic, and weather, they alternate from one strategy to another and use combinations and variations of techniques.

The art of hypermiling is in recognizing the optimum techniques for the situation, and anticipating the upcoming situation, and setting up speed, SOC, and traffic position to best take advantage of it.

I don't do extreme hypermiling, but I do keep in mind that the car reacts mostly to what has happened in the past, and it is up to me to anticipate upcoming situations. Generally I just do simple basic things like saving or using up SOC for upcoming situations, and sometimes coaxing or forcing EV mode or ICE startup if the car doesn't recognize the situation by itself.

So the answer to your question is to alternate between ICE and EV. This is what your car wants to do because it knows the most efficient mode. Don't fight what the car wants to do. Work with the car to help it do what it wants to do.

What your car is trying to do, is to use the ICE when higher power is required, and to use EV when lower power is required. At higher power levels, the ICE is more efficient, and the battery can be charged more efficiently to provide more EV range for when lower power is required. At lower power levels, EV is more efficient.

(Most efficient of all is the powerless glide, which should used at every good oportunity, but only when it can be done without hindering, endangering or provoking anybody.)

You can help your car to do what it wants by anticipating future EV conditions, and allowing or coaxing the ICE to run and charge the battery under conditions that require higher power, such as slight uphill grades and when accelerating, so that it has plenty of SOC for when it wants to EV in the lower power requirement situations.

Or, in anticipation of a good charging opportunity, you can help your car stay in EV as long as it wants, by not doing anything to provoke an ICE start.

Most of the time, the Prius does a pretty good job of recognizing conditions and choosing the optimum mode at the right time. But since it is blind to the future, sometimes it can get out of sync. For example, once in a while it will choose to EV up a hill, run out of SOC at the top, and start the ICE to charge the battery while almost coasting down the other side. If you are in tune with what your car wants, then you can help it optimize its timing and decision making.

 

Driving at 35ish or less for pure EV mode can really help your MPG, but only if you never get to where the ICE has to charge the battery while you're driving that slow.

This means that short jaunts at EV speeds will help you a lot, as long as you can recharge your battery back up with regenerative braking or through the ICE at highway speeds.

I have had trips where my time at 35 MPH was very short, and it always boosted my MPG. There were other trips where I spent more time at 35 MPH and then my ICE would turn on no matter how slow I was going and drop me to very low MPGs which lowered my average significantly.

The bottom line: Using EV only speeds are the best option as long as you keep them to a minimum and not drain your battery in the process, otherwise, stick to low usage of ICE and let the battery assist the ICE when it's at its most inefficient.