Most efficient speed for best MPG?

[Danny Hamilton]

Originally Posted by fthorn

Got to try out the 30 mph on the highway fuel economy today. Storm had everyone driving slow. I did very low speeds (for the highway) and got worse fuel economy than when I cruise at 55 - 60 or so. . . . .

I suspect that the rain and winds had more to do with your poor mileage than your speed. Not reaching S4 might have contributed a bit, but probably not as much as the weather. Did the traffic conditions require you to use the brake much? If so that probably ate into your fuel economy as well.

[bwilson4web]

Originally Posted by fthorn

...
Does it require or must one completely abstain, from putting it into park during that auto-shutdown 'car stopped / no motion' mode?

Hopefully Ken@Japan will chime in but what I've noticed is:

• S3 mode - the engine will auto-shutdown if at least 32+ mph has been reached and the vehicle doesn't need the ICE or deceleration. This can give a false sense of S4 because you can come to a complete stop and the ICE will be off. But if you accelerate and don't reach at least 32+ mph, the ICE will continue to run and run and run at lower speeds regardless of what you do.
• S4 mode - just come to a complete stop and if the ICE is not running, I pulse the accelerator long enough to get it spinning. If it can go into S4, the ICE will stop within ~8 seconds. Then you actually are in S4. It is something I do as part of my daily routine.

GOOD LUCK!
Bob Wilson

[rsforkner]

I think I am going to have to re-read this whole thread as well as some other sources several times. I may need to summarize things to get it all straight in my head. Especially the S3, S4 thing.

I have a flat, straight stretch of city road that frequently has little or no traffic. It's about 3/4 mile long from light to light. almost two if the light is green when I get there. I have tried speeds, with CC, of 30, 35, 40, 45, and 50 (50 is over the limit). I regularly get the best MPG at 45 - 50 MPH. It always seemed a little strange but hey, the car is smarter than me.

Often it goes into EV for a variety of reasons and of course that throws everything off as I am trying to get results with the ICE running.

Time for study,
Bob

[Danny Hamilton]

Originally Posted by rsforkner

. . . .Often it goes into EV for a variety of reasons and of course that throws everything off as I am trying to get results with the ICE running. . . .

Bah, a significant part of why the Prius gets such great MPG at slower speeds is that it shuts the ICE when it isn't needed.

If you force the ICE to idle while you drive slower and slower, then obviously your fuel economy will suffer more at the slower speeds. The ICE continues to burn through the same amount of fuel per minute while idling, and the vehicle covers an ever shorter distance per minute. Divide shorter distance by same fuel and you get a reduced MPG.

Now if you shut the ICE off while traveling some distance you have the opposite effect. You are increasing the distance covered while burning no additional fuel at all. Therefore MPG increases.

What are you trying to calculate? The amount of fuel burned per minute while the engine idles?

[rsforkner]

Originally Posted by Danny Hamilton

If you force the ICE to idle while you drive slower and slower, then obviously your fuel economy will suffer more at the slower speeds. The ICE continues to burn through the same amount of fuel per minute while idling, and the vehicle covers an ever shorter distance per minute. Divide shorter distance by same fuel and you get a reduced MPG.

What are you trying to calculate? The amount of fuel burned per minute while the engine idles?

Agreed. With the Prius let the car handle it. The point is that all over these forums and others I keep reading - Slower = greater MPG. I also read it applies to non-hybrid cars.

In the Prius, I would expect the ICE to run at an optimum RPM. If this is true then at very slow speeds I would expect the ICE to send less energy to the wheels thus lower MPG. I don't think slower always translates to higher MPG.

Thus slow-slower does not always increase MPG. I would expect there to be a "sweet spot", below which you reach the point of diminishing returns. The great thing about the Prius is that it minimizes the problem by using stored energy from the battery to keep the MPG high.

Bob

[Shadestrades]

43-47 mph seems to be the best for me. I regularly see the MPG (when on CC) at 50-70 MPG.

[ken1784]

Originally Posted by bwilson4web

Hopefully Ken@Japan will chime in but what I've noticed is:

• S3 mode - the engine will auto-shutdown if at least 32+ mph has been reached and the vehicle doesn't need the ICE or deceleration. This can give a false sense of S4 because you can come to a complete stop and the ICE will be off. But if you accelerate and don't reach at least 32+ mph, the ICE will continue to run and run and run at lower speeds regardless of what you do.
• S4 mode - just come to a complete stop and if the ICE is not running, I pulse the accelerator long enough to get it spinning. If it can go into S4, the ICE will stop within ~8 seconds. Then you actually are in S4. It is something I do as part of my daily routine.

GOOD LUCK!
Bob Wilson

Hi Bob,
I think your explanation is perfect.
Anyway, there is a thread regarding to the stages.
Prius Stages of Operation & Operating Mode Availability -A Reference Aid

Ken@Japan

[Danny Hamilton]

Originally Posted by rsforkner

. . . .all over these forums and others I keep reading - Slower = greater MPG. I also read it applies to non-hybrid cars. . . .I would expect there to be a "sweet spot", below which you reach the point of diminishing returns. . . .

Every vehicle has a certain amount of energy used when "on" simply to keep everything operating. In the Prius, since it can shut the ICE off, this base energy use is much lower than in other cars.

As such, every vehicle suffers poor MPG at VERY low speeds because this base energy use is larger than the energy use to move the vehicle. As the vehicle increases in speed, it eventually crosses a threshold where the energy to keep it operating is less than the energy to overcome rolling friction and air resistance and other additional energy drains of a moving vehicle.

Because the Prius can shut the ICE off, it reaches this threshold at lower speeds than other vehicles. Through testing, some of the PriusChat regulars have found this speed for the Prius to be somewhere around 20 MPH. In other words, 20MPH will get you the best possible MPG in a Prius. As you get further away from 20 MPH in either direction (faster or slower) your MPG drops. So, for the Prius, slower increases MPG until about 20 MPH.

On other vehicles the speed at which MPG peaks is higher than the Prius, because the fuel used to keep the ICE idling needs to be compensated for with some reasonable distance covered. This number is probably specific to each individual vehicle, but I'd expect it to be more than 20 MPH and less than 45 MPH for most vehicles. So on expressways, slower will almost always mean better MPG, but on roads where the speed limit is closer to 45 MPH faster might mean better MPG in some vehicles.

Originally Posted by rsforkner

. . . .In the Prius, I would expect the ICE to run at an optimum RPM. If this is true then at very slow speeds I would expect the ICE to send less energy to the wheels thus lower MPG. . . .

Which it does. Of course it eventually reaches a point where the optimum RPM is 0 since even idleing it generates more energy than the wheels need. Actually a more accurate description might be that the optimum RPM is less than the idle RPM. Since the vehicle can't run the ICE at half the RPM, it instead runs it at full RPM but cycles it on and off so that it is only running half the time. The excess energy generated while the ICE is on is buffered up in the battery and fed back to the wheels while the ICE is off to keep the vehicle moving smoothly.

[richard schumacher]

Originally Posted by fthorn

Does it require or must one completely abstain, from putting it into park during that auto-shutdown 'car stopped / no motion' mode?

The only reason to shift to "P" during a stop is because your foot gets tired of pressing the brake pedal. As long as you press the brake hard enough to stop the arrows on the "Energy" display, from a fuel economy perspective it is equivalent to being in "P".

In the US the most fuel-efficient speed is 42 MPH. In Europe, Japan, and Australia it's 42 kph.
:_>

[rsforkner]

Originally Posted by Danny Hamilton

Every vehicle has a certain amount of energy used when "on" simply to keep everything operating. In the Prius, since it can shut the ICE off, this base energy use is much lower than in other cars.

I'm in agreement with all of these points. The Prius does a much better job managing its available energy and thus has great FE.

Originally Posted by Danny Hamilton

Of course it eventually reaches a point where the optimum RPM is 0 since even idling it generates more energy than the wheels need. Actually a more accurate description might be that the optimum RPM is less than the idle RPM.

It all boils down to the fact that the Prius is a GREAT car.

I did have another observation/question but I think I'll start a new thread regarding ICE vs battery at highway speeds.

Thanks,
Bob