MPH versus MPG curve

:welcome: Welcome to PriusChat, PineBluffPrius! Um, since the Prius has continuously variable power output from three sources through the constantly engaged Power Split Device, what gear range are you referring to? Not trying to be a smart @ss, but there isn't a simple curve. There are many threads here on PC you can search for discussing fuel efficiency, hypermiling, and pulse & glide.

 

I have the earlier model NHW11, 2001, and have two such charts:


This chart is based upon more measurements and a drag derived, math model:

Your NHW20 should have somewhat better performance. If you use the ratio of EPA numbers, it should be fairly close. You might be interested in my test results comparing pulse and glide versus cruise control at 34 mph.

Over the same, flat, 2.5 mile route, my pulse and glide protocol consisted of:

1. low speed limit is 25 mph
2. 'resume' cruise control acceleration to 43 mph
3. 'pause' cruise control to coast to 41 mph and ICE stop
4. use "N" glide to 25 mph

The average speed of this Pulse and Glide protocol is ~34 mph so I interspersed cruise control managed runs with the Pulse and Glide runs.

Overall, as the chart shows, there was an 11% improvement with the 25-43P&G over steady speed 34 mph. I was testing on a route that had no other traffic. I also tried P&G centered around the maximum range speed, 18-20 mph, and found P&G had worse performance compared to holding the speed constant over this same, flat, 2.5 mile track.

Have you considered making your own measurements?

Bob Wilson

 

After extensive testing, PC member Mwbueno (aka Wayne Brown) has plotted the relationship. See this for the curve and background.

 

Yesterday I drove about 200 miles from central to southern Florida. Just by watching the MFD (and the road) I believe my observations are pretty close to your chart (excellent by the way).

Somewhere between 70 (the speed limit) and 75 there was a significant drop in MPG from just below 50 to around 40 or so. All under curse control. I tried to find that elusive "sweet spot" but was unable to do so due to traffic conditions.

From those observations I concluded that 48-50 at 70 and 40-43 at 75 what what it was. That coincides with your charts very close.

Thanks for all the work.

Bob - cursin' at 70 with cruse control

 

I would agree with the consumptions at 70mph, interesting to note that 40 and 50mph speeds yield good mpgs, which I will try when I dont annoy other motorists doing it

 

There is no doubt an inverse relationship between speed and MGP. Lowering your speed will raise your gas mileage. The trick is to find the speed which is comfortable, reasonable, acceptable to yourself and those around you.
For interstate driving, at steady speeds, in my quest for excellent gas mileage, I favor 60 MPH. It results in about 60 MPG for me.
I would not drive 60 MPH in the far left lane, that's suicide, but in the far right lane, it's quite acceptable. I've actually noticed lately, more and more folks over there with me driving at more conservative speeds. Many pick-ups and SUV's! Not just Granny, and me on my quest anymore.

What I have found, that really surprises me, is the difference in fuel economy I can get by simply by slowing from 60 to 59 MPH. Yeah, just slowing 1 MPH, from 60 to 59 MPH results in astoundingly better averages overall, and 5 min intervals. I see blocks darn near 70 MPG on the interstate at 59, and no where near that 60 MPH>
I'm sitting at 62.1 MPG as I speek, just getting in from my 100 mile R/t commute. Same R/T at 60 MPH would result in closer to 58-59 MPG. I've seen this repeatedly.

 

41 MPH Is the Magic Number

I have done some very intensive testing over the last 3 months I have owned my Prius. Here are my results:

My 2008 P5 Prius has just about 2600 miles on it now. This test was done 4 times to achieve an average of 73MPG on the car w/o any modifications.

I drove home from my parents house at night and instead of taking I95 as I normally would go (the quickest route), I took some new city streets that have very little cars on them. Very minimal traffic lights. All tests were performed with temp in the low 70's, at night with my HID headlamps on, radio on, interior dash fairly dim and NO AC. No other accessories other than a portable DVD player was charging off the 12v system. The road is pretty much level the entire route and I use cruise control at exactly 41 mph. When I anticipate having to stop, I release cruise control and either let the car slow on its own or glide based upon when I feel I will need to stop as I try to keep the car rolling whenever possible to avoid expending energy to have to get back on track. I wish I had a camera with me to take a photo of the MFD as the curve started at 73 curved up to 89 and back down to 73mpg over a 45 minute ride. I think that is AWESOME! So since these tests, I drive around my city at no more than 42mph and I see my average MPG start to climb. I use cruise whenever possible and again, try to time every stop so I don't have to. I find that really saves fuel.

I will be doing my highway tests soon. I will post once they are completed.

I have saved over $600 in gas in less than 3 months since i got rid of my Toyota 4 Runner 6cy. SUV! :rockon:, yet I still find myself bitching at the pump prices. I guess I won't be happy until I never have to pay for fuel again and that is not far off!

 

As much as I want to hit 50mpg, I can't stand driving slower than 70mph on the interstate

 

Then your only alternative is to reduce your coefficient of drag:

• wheel well covers - this blocks air bleeding out of the wheel wells. Look at an Insight to see an example or a French car.
• air dam and side skirts - this reduces the amount of air that passes under the car so it passes around the top and sides efficiently.
• variable cooling air inlet - this should be tied to the coolant temperature and open up as the temperature exceeds 75C. This will reduce cooling air losses.
• tuft study - tape small strings every square foot of your vehicle and using a USB or equivalent camera, make movies or photos of the strings and their behavior at your favorite cruise speed. You'll need to fabricate some sort of frame to hold the camera. Look for areas of flow separation and test foam and duct-tape shapes to reduce the flow separation.
• low rolling resistance tires - narrow, larger diameter, low rolling resistance tires to reduce profile air drag and rubber-to-road hysteresis losses. Get 51 psi tires and run at the pressure (aka., Sumitomo.)
• transaxle oil - yours may be suffering early aging due to severe, high-speed service. A more frequent change interval may be called for.

GOOD LUCK!
Bob Wilson

 

You forgot weight reduction by removing passenger seats, insulation, air bags, floor mats, going on an all liquid diet, etc. :spy:

 

Not really. In differential equations we call the potential-kinetic energy laws of motion a 'conservative' field. This means if you can stay off the brakes and minimize regenerative energy cycling, there is no energy loss and the vehicle mass really doesn't enter into the equations. It just takes careful terrain driving: (1) start at rest from high spot, (2) travel at a lower altitude, and (3) always climb to the apex of a hill to stop. Piece of cake!

There is a slight weight penalty on rolling drag but start with your vehicle empty weight and then calculate the nominal effect of gasoline, oil, seats, spare tire, body spare tire, seats, clothing, safety equipment ... The percentage change will be pretty small and easily handled by accidentally making the tire pressure 1 psi higher.

Bob Wilson
ps. Don't make me throw my math at you!:car:

 

Re: 41 MPH Is the Magic Number

I drove home from my parents house at night and instead of taking I95 as I normally would go (the quickest route), I took some new city streets that have very little cars on them.

What kind were the very little cars? Smart cars? Beetles?

 

To add/expand to your list of things to reduce aero drag:

• Automatic grill shutter - Air flowing through the engine compartment is typically a third of the car's total drag. A grille which opens/closes based on engine cooling requirements would improve engine warmup time and reduce drag at speed. I think that the top grille could probably be permanently closed - but don't know if anyone has torture tested this setup.
• Completely flat underbody - Look at the bottom of a Tesla or Lotus, they are completely flat with no protrusions. Ideally the rear would gently taper upwards at an angle of a few degrees with the rear as high as possible. A completely flat underbody would also reduce the need for a front airdam who's purpose is to keep air from passing under what used to be a turbulent underbody.
• Eliminate side mirrors - replace with cameras and screens.
• Eliminate rear window wiper
• Tape body seams smooth
• Lower ride height - (look at what how low the salt-flats top speed guys run)

 

bob, I bet you that I could easily get 50mpg if I somehow shed 1000lbs of weight from my prius! but what do i know, I'm just an EE. I usually only have to deal with electromagnetics & electron not mass transport or fluid dynamics :mod:

 

Ichiro, Bob was saying that weight does not make a significant difference on flat highway at a constant speed. Now throw some hills or start varying the speed some, then yes, weight does make a significant difference in fuel economy.

 

I know, I got his point ... I'm never going to get much better FE than I am now unless I target a lower avg highway speed. I was just kidding around about the weight reduction. Personally, I like the idea of shedding dead weight for other reasons (handling and braking).

 

If you manually change the speed by using the accelerator, brake or regeneration, it makes a difference. If you let speed and altitude vary without any other force, weight has a minor impact from rolling resistance. Think of a pendulum.

Once a pendulum starts swinging, it will smoothly come to complete stop, reverse direction and travel at nearly the same speed in the reverse direction. Your car at the top of a hill with a valley in the middle will attempt to do the same thing minus the rolling and air drag. Only rolling drag has a weight component and it is relatively small.

GOOD LUCK!
Bob Wilson