FuelEconomy.gov official ratings posted - 58 City/53 Hwy Prius Eco - 54 City / 50 Hwy for Prius

The Two Eco has a higher energy dense Lithium-Ion battery, a specially coated windshield to reduce load on the A/C and special higher pressure lower rolling resistance tires that are part of the MPG equation. My argument is that an option to retain a spare would not cost too much in MPGs since most of the difference between the Two and the Two Eco is gleaned from the other enhancements.

 

and no rear wiper.

 

But the Eco badge on the rear adds about 1 oz. It partially cancels out the wiper delete.

A compact spare weighs in at 28 lbs.

The 65 lb figure came from deleting the spare and the lesser weight of the Li-ion pack.

The Eco Two tires are not higher pressure lower rolling resistance tires. They are about the same as what is offered on the other packages with 15" tires. For the record, they are Bridgestone Ecopia 422 Plus, 44 psi max, treadwear 600. Although they have been proven to hold significantly higher pressures than 44 psi, in my case.

 

LOL.

 

According to Car and Driver, the special tires are "Dunlop Enasave 01 A/S low rolling resitance tires inflated to slightly higher pressures", not the ones you cite. Maybe both of these types are being used. But you're right, part of the 65 lb. saving is the Li-Ion pack; I wasn't reading it right. Deleting the rear wiper is also a problem for me; it's a must have in a deluge. So maybe I'll have to pick one of the other models, after all.

 

maybe toyota used special tyres for the mpg testing, but isn't putting them on the cars.

 

I agree that the C&D Prius that was tested had dunlops, but I haven't heard that any of the production Two Eco's have them. Even if both are being used, there can't be much difference between them, given the same EPA rating.

 

I think you said it best, the average buyer will not care about the very slight added efficiency of the Eco Two but the Eco warriors in here do and that's why different models are offered. Average buyers are motivated more by costs and not about how much less CO2 emissions are produced.

 

Agreed, average buyers are motivated by cost, and the Two Eco offers the lowest cost model that offers a Li-Ion battery. It is the only model that has the heat reflecting windshield and the nifty Eco badge. The higher MPG's are bonus. If you want old NiMH technology, you can save $500 with the Two.

 

Isn't there a thread here trying to track the different tire models that people are getting?

 

If not, there should be.

 

The three tire sizes: Need your help! Prius 4 tire information spreadsheet | Page 2 | PriusChat

Thanks!

	Tire model	psi	rev/mi	lbs
1	P195/65R15 89S	36	833	19.6	1.19% slower; true MPG > indicated
2	P195/65R15 91H	32	843	21.1
3	P215/45R17 87V	33	845	23.5

Typical wheel weights:

• 15" AR95 Estrella - 17.2 lbs
• 17" AR95 Estrella - 23.0 lbs

So minimum and maximum weights:

• (19.6+17.2) * 4 = 147.2 lbs
• (23.5+23.0) * 4 = 186 lbs

Not calculated is the polar moment of inertia, the rotational inertia. However, the relevant formula has 'd**4' the fourth power of the diameter. Then there is the mass of the wheel and tire. What this means is swapping the lightest 15" wheel and tire for the 17" wheel and tire is going to slow the car acceleration and require more gas-burning power to accelerate or brake.

Bob Wilson

 

Why would changed tire pressure make such a big difference in rev/mile?

Correct me if I'm wrong, under inflated tire does not have circumference according to radius from center of the wheel to the ground. There is a change in rev/mile with different tire pressure but as far as I know the change is minimal.

I found a video to back up my statement

 

At 1:59 in the video,"The rolling circumference of a wheel therefore does not change significantly when the tire pressure is changed." A nice demo which at those speeds is close. However, there are other experiments with different results: http://techno-fandom.org/~hobbit/cars/tpms/

In effect, this device displays DIFFERENCES between the rotational speeds
of any two adjacent wheels around the car, as a beat frequency from the
ABS speed-sensors mixed together. It is shaped like a diamond because
each LED represents a side or end between two wheels. The topmost LED
displays the spin-speed difference between the front wheels, for example,
and the remainder positionally correspond to the other pairs in an obvious
way. An immediate objection may be that many circumstances of normal driving
will produce major deltas, such as taking a turn or even accelerating or
decelerating, but under "float" conditions on the straight and level the
green diamond should show only very slow changes in any LED's brightness.
It is then up to the human brain of the driver to interpret any anomalies.
The single low-tire warning is any two adjacent LEDs changing brightness
faster than the others, as the wheel in question turns faster.
. . .
At that point, switching the scope to "A + B" channel mode produced a sine wave
with varying amplitude as the two sensor signals drifted past each other and
added or canceled, and there, in a way, was my representation of what the
LEDs should do. That first very simple test setup coupled with letting some
air out of one tire taught me that the whole thing was workable at all and
gave rise to those early reports.
. . .
Going from 44 PSI down to Toyota's recommended 33 in one tire produces a
*very* visible issue with the two LEDs bordering that wheel position -- a
blink rate of as much as 2 Hz, which says to the astute observer, "FIX me
FIX me FIX me!" And while 30-something PSI is still reasonably safe, it's not
good for fuel economy. But seeing a persistent anomaly like that definitely
gives the viewer more motivation to fish out the tire pump and deal with it.
. . .
Parking or even driving with one side of the car in direct sunlight may
produce a fairly significant difference between the sides, and look like a
continual turn even when going straight until the tire temps equalize out
again. If the condition persists past that, of course, stop and check for
*two* soft tires…​

Good Prius friend Hobbit did excellent work but a clue to why this is the case comes from one of my experiments on tire pressure and tire temperature: Experimenting with Tire Pressure? | PriusChat



BACKGROUND

The tire sensors are multi-toothed which means their signal is "N*tire_rpm" where "N" is the number of teeth in the rotational sensor. So at 60 mph, the tire speed sensor will be making ~850 revs/minute times the sensor teeth count, say 10, in the neighborhood of 8,500 Hz. However, a low-pressure tire will heat up significantly and the thermal expansion will change the tire diameter. Good Prius friend Hobbit is measuring the change in diameter driven by the temperature change and has already documented sun/shade effects.

The video, Hobbit's hack, and my metrics are consistent with physics and tire temperature effects. Just the video is at tire speeds too slow to generate significant heating effects. Hobbit's hack works on the tire temperature which my direct tire metrics shows is significant.

Bob Wilson

 

I knew you had a thorough explanation ready

 

I find it difficult to believe that inflation pressure of a modern tyre changes the distance covered per rotation. The reason is the tyre -tread- is what controls that, and it doesn't expand with inflation pressure except on very poorly made tyres.
What inflation pressure does control is "bump absorption", or suspension "roughness". It's actually a small part of the suspension (the tyre sidewall, that is).

If inflation pressure were changing the distance traveled, the tyre would wear out very fast, as the tread would be slipping on the pavement continuously.

 

Explain, I do not follow this at all.

 

The tread is a fixed length, and is very strong and won't expand in length. Yes, it is wrapped in a circle, but the circumference is fixed. If changing the inflation pressure changed the effective diameter then the tread, with a fixed length would have to slip on the pavement.

 

I think we will have to disagree.

 

Need your help! Prius 4 tire information spreadsheet | PriusChat
That's the thread I was thinking of. People aren't always getting OEM model tire with their new Prius. This is something to consider when judging a new owner's fuel economy.

Higher tire pressures help by decreasing the tire's rolling resistance. Which isn't all due to the friction between the tire and the road. The other element is sidewall flex; the bulging out of the portion of the tire at the ground. As the tire rolls, energy is lost to push out the side wall as the tire rolls down to the pavement and under the car's weight. Since we don't use superball rubber to make tires, very little of that energy is recovered then the sidewall snaps back into place as the tire rolls on.

Bob mentioned low inflated tires heating. That is because the energy lost to the sidewall flex is going to heat up the tire. This extra heat build up is why an under inflated tire is more likely to blow out than a properly or over inflated one. The higher pressures make the sidewall more rigid. The less flexing, the less energy lost. The stiffer sidewalls also improve cornering on the tire.

I've seen some reduced traction on wet pavement that could be attributed to higher tire pressures, but the biggest trade off to higher tire pressures for many will be rougher ride.

The tire's circumference isn't a true circle once it is supporting the car's weight. The contact patch is being squished flat, cutting across an arc of the circle. The tread there is being compressed together; less space between the ridges. Too large of a contact patch, and you'll get abnormal wear patterns in regular driving. Increase the sidewall rigidity, and the contact patch gets smaller, with a smaller arc cut across from the circle.

tl;dr The tire tread circumference doesn't change with tire pressure, but less of it is squooshed down with higher pressures. So the effective circumference increases.