High speed mpg

I suspect I am telling more than I know, but I seem to remember someone posting a graph showing gen 3 efficiency peaks at 45 and again at 75 mph ??

 

That would be closer to 54 mpg US. And because the display tends to read high, the real mpg is likely several counts lower.

Not that we know about.

But many other factors could contribute to what you saw, and it would take considerable effort to control for or separate out all of them. Weather (breeze direction and speed), pavement condition, small and even imperceptible changes in road slope, changes in engine temperature (if not yet fully warmed), changes in traction battery charging or discharging, ignition timing under changing loads, climate control operation, etc., all contribute to mpg.

If the changes you saw are really rooted in some of these other factors, then I'd guess that as you experiment more, the observed changes not be fixed to particular speeds, but will shift around from test to test.

 

unless you're on a perfectly flat road with no winds, it's difficult to know. speedo is off 1-2 mph here too.

 

I know you said flat but maybe it was slightly going downhill? I've done 80mph (in cruise control) with my MPG readout close to 100.

 

Re: whether or not it was downhill, Google Earth can give you an elevation profile of a route between 2 points! | PriusChat can help one confirm.

Long ago, I confirmed why I'd see higher iMPG going one direction down this expressway I take than the other... it was due to a gradual, but not very noticeable elevation difference.

 

The responses suggesting that you might have not been on the flat is because there is a lot of accumulated evidence that the probably isn't such a speed that is a 'sweet spot' for the Prius.

The faster you go, the higher your consumption will be. From around 40 - 45 mph upwards, air resistance will be the dominant force that burning fuel is used to overcome.


iPhone ?

 

Methodology is important because it allows others to replicate the observation. That is why I use trip meter values over a 10 mile route under cruise control. I run two passes in opposite directions over the same routes. The tripmeter values do not include acceleration and deceleration. Then I average the two passes. Later, I use tank values to get a calibration factor for the indicated MPG.

When I replaced the original tires, I found both GPS and mile marker calibration offsets corresponded with the indicated vs tank calibration offset.

I am interested in looking for a way to replicate your observations. Perhaps using a video camera or an OBD recorder. Could you provide more details such as duration of the indicated MPG values?

Bob Wilson

 

Instantaneous fuel efficiency (MPG or L/100km) is misleading.

The overall average MPG over several gallons/liters of gasoline (which Bob Wilson has provided) is a better fuel efficiency metric.

If the Prius has enough momentum (e.g. from going 80mph with a tailwind) and slows down to 74 mph - there will be a short period of time where the instantanous fuel efficiency will be very high before the Prius slows down - When going down a long downhill roadway the Prius will shutdown its ICE and use its momentum instead of requiring addition energy from the power plant. However, unless your Prius is drafting behind a very big vehicle OR your Prius is going down a steep hill or mountain side - the Prius will slow down (especially on a flat road or uphill road) without additional power. At high speeds with cruise control off, the Prius will naturally slow itself down automatically if you take your foot off the accelerator by applying the regenerative braking system - the Prius regenerative braking will recharge the HV battery ( but when the Prius reaches its maximum State of Charge(SOC) of about 75% - it shuts down the ICE and goes on electric vehicle mode 100% of the time until the SOC drops below about 70%). However, regenerative braking is a lossy energy conversion (between 50% to 90% of the kinetic energy is lost as heat during this energy conversion) and the Prius likely won't get the best fuel efficiency when the Prius regenerative braking system is used to convert the Prius' kinetic energy (speed) to electrical energy. Unless the Prius is going over 65 mph (this assumes the Prius is not driving into the wind, aka head wind, or driving with the wind , aka tail wind ) - the loss from aerodynamic drag is much less than the loss from regenerative braking - preventing regenerative braking at high speed and driving lower than 65 mph is key to getting exceptionally high fuel efficiency on the Prius at very high speeds. This means to get the best fuel efficiency on a Prius at very high speeds the driver may not use cruise control, the driver must keep pressure on the accelerator, the driver must try to keep the Prius' speed under 65 mph, and the driver must monitor the HSI display to prevent regenerative braking from occuring. This can be very challenging on hilly or twisty roads.

 

Here is what I plotted on the first tank driving from the dealer where we picked it up:

These were indicated until we measured the calibration factor. Subsequently, we found the tire diameter explained the calibration scale.

Bob Wilson

 

Bob, are you able to comment on whether those apparent 'bumps' at 60 and 75 MPH are real or just within the noise level of the measurements? (I'm not sure what algorithm might have been used to draw those lines).

My understanding is that there isn't such a thing as a sweet spot speed for highway cruising, and would be grateful for your opinion on this.

Thanks.


iPhone ?

 

I saw the bumps but did not have instrumentation good enough to further. I did get a miniVCI but it only records for 3 minutes at a time.

I am curious but do not have the tools to go further.

Bob Wilson

 

One of the problem with "hypermiling" claims has been the absence of technical details:

• What was the equivalent speed?
  • If the equivalent speed was 50mph, the "88mpg" means you've driven slower.
• What 'hypermiling' specifications were used?
  • Speed ranges?
  • Acceleration profiles?
  • Accelerator pedal position?

Now I did this without using 'hypermiling' techniques because I documented how:

• cruise control speed set 26-27 mph - notice average was still "22mph" due to stop lights, signs, traffic.
• shift into "N" anytime possible while maintaining 26-27 mph - in particular, looking for engine OFF
• minimum 1 hour for each trip - ensures the warm-up penalty is diluted by 100 MPG operation

Driving slower can easily achieve "88mpg." In fact, driving slowly can and does easily exceed "99.9mpg." For example, I did this in 2007 in a 2003 Prius:


I've also investigated Pulse and Glide. For example, using a speed range of 25-41 mph, I compared the cruise control and P&G equivalent speed (aka., block times.):

The equivalent MPG:

So Pulse and Glide 'works' provided you are willing to accept the higher, manual overhead of trying to use it in traffic. Even the SAE paper describing their testing says it should only be done on a close track . . . not in traffic. Some of us think safety is more important than an 11% fuel savings.

Bob Wilson

 

if you want to understand where or why your Prius is more fuel efficiency in one instance or another
you will need to tighten up your methodology - and record more different types of data points.
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1. you have to exactly identify the road -route you took via google-earth to identify any elevation changes in the road. Why? Changes in elevation (gravity) can effect the overall fuel efficiency of a motor vehicle. Changes in momentum (like how sharp a turn is on a particular road/route) as well as how smooth the road surface you have selected is can effect the fuel efficiency of your vehicle .

2. You also need to identify the time and type of weather your Prius is travelling in. Wind direction and wind speed can effect fuel efficiency. Road surface condition can also affect fuel efficiency. The Prius ICE energy efficiency drops when the outside temperature falls below 50 degrees fahrenheit and when the road surfaces are wet or icy. Hence, you need to pay closer atttention to driving environmental factors.

3. You also need to describe the traffic conditions while you were driving and whether you drafted (drove closely behind another vehicle) too. Drafting is consider unsafe on roads where frequent stops or slow downs occur. Drafting can be done with some margin of safety when the lead vehicle is in constant radio contact with all following vehicles.

4. When gathering experimental data - you need to use a cold engine since a warm engine will give the Prius a variable and often unpredictable 10% to 20% MPG boost from the very start ( its like using an Engine Block Heater ).

5. The problem with using cruise control is that it will apply regenerative braking to slow down the car and regenerative braking loses 90% to 50% of all the kinetic energy as heat when trying to making electricity to store - and fuel efficiency wise its better to go a faster and lose a tad more from aerodynamic drag than to use regenerative brakings loss - if driving safety and/or getting a speeding ticket isn't an issue. This is not to say regenerative braking is bad - when you need to slowdown or stop - it can recover energy that would have been lost as heat - but it is a poor fuel efficiency substitute for actively managing the vehicles speed. Cruise control isn't terribly bad but it isn't the most fuel efficient way to drive - IMHO Cruise control best feature is that helps reduce driver fatigue when one is driving all day long.


BTW: There is a thread on Prius Chat's Gen3 Forum for posting the HSI display for a trip (IIRC it must be greater than 10 miles) where it reads 99.9 mpg.

 

I'm OK with 50% but the 90% is a bit excessive. Under cruise control, it is fairly modest in the energy levels so 50% is much closer to the observed values. I'm more concerned with the charging because it is exothermic, it makes our NiMH batteries hotter.

I would only add that I prefer to shift into "N" if I want to avoid regenerative losses BUT I grew up on manual transmission cars.

Bob Wilson

 

I just finished a 900+ mile road trip. It was all on the freeway, with the speed limits about half-and-half 65mph and 70mph. I am posting the final screen shot. I was able to get up to 60mpg in the 65mph zone, but not when the speed increased to 70mph. I drove at or above the limits and used the cruise control very sparingly. I used N not at all. I think the post about cruise and regen is right on. You have to stay out of regen unless there is a lengthy hill.

 

This is the end of trip screen on the PiP while driving through Virginia. No EV was used, it was all HV.