What is absolute best speed for highest mpg?

If you are running out of gas and have no traffic you want to

1. turn off the fan (disabling AC/heat)
2. turn off the radio (especially satalite radio or aftermarket amplifiers)
3. turn off the headlights (if it is bright enough to safely do so)
4. stay near a speed low enough to be able to run electric only.

With my gen II if the battery is in the mid bars the gas engine kicks in on nearly level ground around 20 mph so with a slight downhill I can do more than 20, on a slight uphill less than 20.

Either way find that spot where the gas engine kicks in and travel right at or below that speed. That is the speed where wind resistance kicks in more than rolling resistance and the gas engine kicks in at that level to keep the battery from having to discharge too fast.

Too far below that and you spend more time charging/discharging the battery and if it is night time you have more vampire load over time vs distance traveled (dash, headlights, etc).

Too far above that speed and you incur more wind resistance. Enough so that it will offset any charging / discharging losses from running on battery only.

If you get the speed just right and all the other variables like slope and temperature are in your favor you'll get something on the order of 150 miles to a gallon. (If you have to stop and go think more like 100 MPG, if it is cold outside you may drop as low as 50 MPG even with the heat off because the catalytic converter will demand the engine run just to heat it).

If you aren't at the OMG I'm out of gas level then kick it up a few MPH above that and cruise at 25 or 30 MPH or consider trying to pulse and glide from a higher speed like say 40 down to 25 and back up to 40. Myself if I was low on gas I'd be more worried about watching my mirrors, navigating to the gas station, watching for danger so I'd probably give up on the P&G and go for steady speed when crunch time hits.

 

The blue line would be he MPG without any fixed load, the grey line is the MPG with a 511 watt fixed load. The read line is the HP needed to offset air resistance.

Generating accurate graphs is not trivial, Bob Wilson does as well as any on Prius chat.
Here is a graph by Frank Tiger of OBDII mileage.



Graphics of average fuel economy vs speed | PriusChat

(this, unlike Bob's theoretically best mileage, is Frank's actual mileage over 1 hundred thousand data points. You could do better or worse depending on driving skill)

My graph is way less on topic: https://scontent-b-dfw.xx.fbcdn.net/hphotos-prn1/37627_1542805172434_4766290_n.jpg

 

His peak near 40 mph is near the point where the Prius won't let you run electric only on level to downhill grades. I doubt it is related to the peak for the gas engine I would say it is an anomaly due to high state of charge from prior regeneration.

If you want highest MPG on flat ground with steady speed it will be much closer to 20 mph. If you have the time to test I'd start at 10 mph and go up in 2 or 5 mph increments until you reach 40 mph. I think you'll find your peak below 25 mph.

 

You would have to know Frank's typical cruise speeds, for me, I cruise at 61 and 48 MPH a LOT, so my MPG for other speeds would mostly accelerating though that speed, while MPG at 48 and 61 would be steady state.

 

A British PC member pointed out that a steady speed of about 35 MPH yields 90MPG or so. The ICE comes on periodically to re-charge the battery. Using the battery advantageously can still results in big numbers.

 

The chart may be right. My best ever fuel economy was filling fuel and resetting the trip meter on entering a state park where speed is limited to 35 mph. The 7+ mile loop in the park with speed never exceeding 35 mpg indicated 95 mpg average.