Why low mpg on 2013 prius 3?

I have picked up a few over the years:


Then it turns out as the speed increases, there is a non-linear effect:

The paper described vibration modes that as the speed increases becomes a significant tire energy drain. But these papers were to establish an understanding of what is going on.

I wanted to find out what was happening to my Sumitomo T4 tires as a function of speed and inflation pressure. The protocol was:

• start with tires at maximum pressure
• drive a non-trivial distance, 5 miles or more
• quickly pull off to a safe shoulder, the off-ramp of the highway
• measure the tire tread temperatures
• reduce pressure and repeat

Sad to say, I had different speeds as well as pressures so the chart is a little confusing:

There are two variables, speed and tire pressure:

	Column 1
0	[th]pressure[th]speed[th]temp[th]Comment
1	[tr][td2]44 psi[td2]65 mph[td2] 9F[td]driver side #1
2	[tr][td2]44 psi[td2]53 mph[td2] 5F[td]driver side #1
3	[tr][td2]44 psi[td2]65 mph[td2] 2F[td]driver side #2
4	[tr][td2]44 psi[td2]53 mph[td2] 0F[td]driver side #2
5	[tr][td2]44 psi[td2]65 mph[td2] 8F[td]driver side #3
6	[tr][td2]44 psi[td2]53 mph[td2] 5F[td]driver side #3
7	[tr][td2]44 psi[td2]65 mph[td2] 4F[td]driver side #4
8	[tr][td2]44 psi[td2]53 mph[td2] 3F[td]driver side #4
9	[tr][td2]44 psi[td2]65 mph[td2] 9F[td]driver side #5
10	[tr][td2]44 psi[td2]53 mph[td2] 4F[td]driver side #5

A 12 mph speed reduction gives a significant reduction in tire temperature. Less energy is lost to tire drag. This happens to be the knee in the curve of one of the earlier studies:

• 53 mph = 85 kph
• 65 mph = 104 kph

There are two variables, speed and tire pressure:

	Column 1
0	[th]pressure[th]speed[th]temp[th]Comment
1	[tr][td2]53 psi[td2]65 mph[td2] 6F[td]passenger side #6
2	[tr][td2]44 psi[td2]62 mph[td2]14F[td]passenger side #6
3	[tr][td2]53 psi[td2]65 mph[td2] 0F[td]passenger side #7
4	[tr][td2]44 psi[td2]62 mph[td2] 1F[td]passenger side #7
5	[tr][td2]53 psi[td2]65 mph[td2] 6F[td]passenger side #8
6	[tr][td2]44 psi[td2]62 mph[td2]10F[td]passenger side #8
7	[tr][td2]53 psi[td2]65 mph[td2] 3F[td]passenger side #9
8	[tr][td2]44 psi[td2]62 mph[td2] 4F[td]passenger side #9
9	[tr][td2]53 psi[td2]65 mph[td2] 2F[td]passenger side #10
10	[tr][td2]44 psi[td2]62 mph[td2] 4F[td]passenger side #10

A 3 mph speed reduction that should cooled the tires along with a 9 psi pressure reduction still gave a significant increase in tire temperature. A lower pressure increases the tire drag.

I'm using an engineering trick, heat, to give a metric of tire resistance. This survey study was looking at the correlation between speed and tire pressure on tire heating and found these correlations:

• higher speeds -> more tire drag energy loss
• higher pressure -> less tire drag energy loss

One surprise was how higher pressures appeared to mitigate wheel misalignment which I was not expecting.

To replicate this study, get a Harbor Freight or equivalent, non-contact temperature probe. Use a highway stretch during a time of fairly constant temperature, +/- 1 hour around a peak minimum or maximum temperature. This typically is around ~1-2 PM in the afternoon or about an hour before dawn. Regardless, make measurements of the pavement temperature before pulling off as the base to calculate the temperature change. Pay attention to traffic.

Bob Wilson

 

i should have known better. <grins>

 

Actually you did but not perhaps in the way expected. My reading indicates tire rolling resistance is a result of hysteresis. If you deform rubber, the amount of energy returned is not the same as was used to deform it. A rubber spring has higher energy loss than a steel spring. But this hysteresis loss is proportional to the deformation.

At higher pressure, the tread deformation is less, your "removing rubber from the road" is accurate. Higher pressures also lead to stiffer sidewalls having less flex. If you remember the Ford/Firestone issue many years ago, the tires were failing because the pressure was too low. The extra heat from the extra flex led to the tire failures and the top-heavy SUVs driven by unskilled drivers became a well documented problem.

Now one of my experiments had mixed results:

The left tire was the first replacement. The front was my first oversize experiment. The one in the middle is the largest that would fits in the smallest wheel well, the rear. Currently:

• middle sized tires are on the rear
• largest sized tires are on the front

I drove up to Hybridfest and back and swapped the middle and largest tires there:

• medium front, large rear - mileage was dead accurate' and handling neutral stability. The car would not track down the road but it wasn't 'darting'. It had neutral stability.
• large front, medium rear - mileage was understated and handling improved straight-line tracking. Later I realized the lower rotation speed was in effect 'over-drive', a 6% effect proportional to the miles/rev.

I am confident the over-drive effect, getting the transmission to turn slower in the final gear ratios and differential, dominates the lower rolling resistance. To test this hypothesis, I would have to swap the rear tires with same size as the front and take careful measurements . . . very careful measurements . . . beyond the precision of my instrumentation. The only variable would be the rear tire diameter.

Bob Wilson

 

Actually, we need to get back on subject:

Our Prius is a gas engine, driven car, and the traction battery and control laws appear to be aimed at maximizing engine efficiency. So what I've found:

• catalytic converter warm-up, ~50 seconds - best done in "P" for minimum fuel burn. The engine will auto-shutoff indicating the engine is now in "closed loop" where the car tunes the mixture.
• coolant warm-up, ~2-3 minutes, 1-2 miles - best done at slowest engine load, say neighborhood speed of 25 mph if an option.
• ordinary driving - use slower routes if available; follow slower traffic (do not tailgate) if available, and; use cruise control to handle speed management for less throttle 'fretting.'

Now the plug-in Prius has enough traction battery energy that the initial drive can let the engine run 'unloaded', the same as if in "P". As soon as the engine is warmed up, save the traction battery energy for later trips and you're in a car that depending upon speed can get outstanding mileage. The original poster's car is not a plug-in.

He needs to use the warm-up mitigation tricks and drag reduction techniques that work with his particular car.

Bob Wilson

 

On highways...in city driving this is not a correct statement. Regen is why the car gets better mileage in the city than on the highway. Even on the highway, you regen every time you ease off on the accelerator. There are a lot of posts about regeneration that are not correct and they need to be carefully examined.

 

It plays a part but the vehicle performance is the sum of multiple effects which includes:

• speed related drag
• efficient engine
• efficient transmission
• tires
• warm-up costs
• control laws
• regen when necessary

They each play a part and the model is quite complex. My studies indicate in urban travel:

• warm-up - easily the largest fuel penalty
• speed - less is best if there is an option
• constant kinetic+potential energy - these transfer with 100% efficiency
• braking is necessary and regenerative braking reduces but does not eliminate the round-trip loss

I've long felt if we could get a measure of how much energy it takes to wear down brake pads, we would have a good estimate of the regenerative energy savings. Definitely, cooler brakes because of regenerative braking is better than the hot brakes of ordinary cars.

But going back to the original poster's questions, we need metrics and details. If he can get something like a miniVCI or Scangauge II and perhaps a GPS recorded route, we'll have metrics, data, to make more informed suggestions. If he can use his tripmeter to conduct experiments, he can also start mapping out what works and doesn't.

Bob Wilson

 

Yes. Try it.

 

Sorry, but I still stand by my statement -- with reasonably efficient driving style, most of the battery recharge will come from the engine, not from braking regeneration. This is especially true with a good DWB (driving without brakes) technique. While regeneration is a great improvement over friction braking, it is still quite inefficient.

No, differing EPA fudge factors are why the EPA City rating is higher than the EPA Hiway rating. The CAFE city and highway numbers are virtually identical. But the EPA scale starts by slashing 22% off the CAFE Hiway test result, but only 10% off the City number. (Additional tests have been averaged in since MY-2008).

For many drivers, the lower average speed of city driving helps net greater MPG. Regeneration also helps, but isn't enough to do the job.

Not I don't, because I usually don't back off that far. Backing off sooner by a smaller amount (to Warp Stealth or slight EV, not to CHG) is more efficient, and is practical much of the time. DWL (driving with load) is better still.

I agree with this statement. That is part of the reason for my earlier reply.

 

I guess the way to really tell is to compare mileages. I use the battery as much as I can, and am currently at 68.2mpg for the current tank (about 500 miles). This is mostly, but not all, suburban driving over hilly roads. The best I have done at freeway speeds has been 57mpg. I was able to get 61mpg on a trip to Victoria using lower speed state highways. The overall efficiency of the regen-battery-motor system is at least double the engine efficiency. There are, as noted above, a lot of things that contribute to the cars efficiency. According to Toyota engineering, the regeneration system is 30% of the overall total.

 

"57mpg at freeway speeds" ? What exactly is your freeway speed?

Anyway, back to sitting still burning gas, basking in your own nasty fumes.
A car warms up quicker with a load on it and the slowest while idling. Period.
If you can document that you save gas by burning some while not moving, prove it.
One guy says he does this sitting in his garage! Good lord, I hope nobody is living in the rest of the house.
That is the nastiest pollution a car can make during the initial cold start of the day.

True, I don't have a horse in this old fashioned race. I have moved on to the next step in hybrid technology.
I used to say a Volt is an EV for the first 46 miles, (what I'm getting) then it turns into a Prius.

I now take that back.

My Volt did something unacceptable the other day. I ran out of EV while in suburban traffic for the first time.
I usually do this on the highway.
It started the engine normally, (you have to watch for it, you can't feel it) and as I came to a stop light the engine kept on running.
So there I was, windows down, basking in my car's own farts.
There needs to be a SW update to make the Volt behave more Prius like in that situation.

If you want really good mpg and use a clean, potentially renewable energy source, there is a car out there for you !!

 

It was an 850 mile round trip with 70mph limits. It also included two mountain passes and the Columbia gorge...both ways. No cruise control. Moderate traffic. Air temp in the 80's. I am still trying the stationary warm-up system. It does not seem to be an intuitive process, but neither does pulse and glide.

 

But, 'What was your speed'?
57 mpg seems pretty,,, optimistic.
Do you do Fuelly?

And again, what bad thing happens if you start moving immediately and turn gas into car motion????
I used to see how far I could get from my driveway before my car started farting that bad stuff,,, NIMBY!!

 

Fuelly won't make anyone a believer. The GenIII is capable of a lot better mileage than most folks here seem to get. bobwilson got 1000 miles out of a tank...admittedly is was not something most people could duplicate. To get the maximum out of the Prius, you need to really examine your driving style. We seem to concentrate on the tricky parts but ignore the basics. Drive in ECO to keep control of the accelerator; never accelerate any further past mid-line than necessary; drive as slowly as traffic will allow; use pulse and glide as much as possible; be patient.

 

I always accelerate to almost the PWR zone and seem to get great results with that approach. I also found it here in the forum originaly posted by ken@japan I think. I of course use other techniques.

best regards

Andrej

 

There is something really important here:

Less than 1% of all Prius owners have OBD instrumentation. So what makes most sense is to explain how to achieve good mileage using the instruments already in the car. If you have the instrumentation, great, but it is not critical to achieving high mileage. The basic functions are there and we do better when we explain how to use it.

Bob Wilson

 

drysider,
So, you won't say what your speed was when you got 57 mpg "at highway speeds".

I came back from a 2500 miles 5 day road trip recently in my Gen3, always +5 over the posted speed limit using GPS speed letting cruise control handle the throttle while I surfed XM.
1/3 of the trip the car had a big load and 2 bikes hanging off a bike rack.
42 mpg for that trip.
For you to get 57 ,,, you were obviously one of those AH traffic obstacle Prius drivers that give the rest of them a bad name. Did you see the youtube where a guy gets to drive a Prius and try to figure out why people drive like that? He thought it was a normal relatively fast car. He concluded it was "that display" that makes them drive like that !!!

True Fuelly won't make anyone a believer. I requires driver input. You should see MyVolt or Volt Stats. No driver input required! That's why there are no Volts on Fuelly,,,, but I digress....

And all this troublesome driving just to save a buck and a half at the pump.
Are you making a bad name for the Prius by driving that way?

The OP was looking for reasoned answers, not extreme examples of what can be coaxed out of a Prius.
I think he is in a normal range and should not worry.

 

Huh?

Is the "Gen3" yours or someone else? I thought you had a Volt, you kept the Prius too?

To get 58 MPG, drive between 60-65 mph on cruise control, flat land, no headwinds. With a tailwind, a little faster. With a headwind or cross wind, a little slower. It is fairly easy to find semitrailer trucks, RVs, or trailer pulling vehicles in these speed ranges.

Bob Wilson

 

Drysider is clearly Trolling. No one can get 57 mpg at highway speeds, it just doesn't happen. In fact, most of my driving is "at highway speeds" and I have a really, really hard time getting 57 mpg. In fact, those times I do get 57 mpg are in the winter time, with colder temperatures, rain, and wind.

57 mpg.....indeed.......*walks out huffily*

 

Whew....a troll no less. And an AH traffic obstacle. I am properly chastened. However, I did get 57mpg. It was on I-90, which is not especially mileage friendly. I found a semi going 70mph and I did a sort of pulse and glide between 65 and 70 behind him. We were passing pretty much everyone in the slow lane. You can't drive 75-80mph and get 57mpg. bobwilsons chart is pretty easy to read. You also can't do it in cruise control, but as I recall, I was able to get 52mpg in cruise for part of the trip.

 

Bob,
Bought the Volt 2 weeks after the big road trip. Nice graph. You say no headwind. Does that also mean no tail wind? I'm sorry, but I question if your "58 mpg @ 60-65 mph".
The Prius has left the building. Technology marches forward!

Drysider,
Thanks for answering the question. Drafting works. Sorry about the AH obstacle comment. You were clearly something else entirely!!
So, when you were getting 52 mpg w/cruise set, at what speed was that? That is more real world useful data than your drafting ways.