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Old 03-23-2010, 09:25 PM   #1
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On tire pressures, the Yaris, and the Prius

Tire pressure is another topic that comes up frequently which I have some thoughts on. As everyone here is no doubt aware, the Toyota Prius is Toyota's Tour de Force of engineering (some would say over-engineering) wizardry designed to eek out every last bit of fuel economy its designers can find a way to manage, and the de facto poster boy of the Green movement. So it might be interesting to look at what Toyota is recommending for tire pressures on their flagship MPG-mobile.

And the answer is interesting. It comes with P195/55/R15 tires rated at 1168lbs maximum load at 44 psi. The curb weight of the Prius is 2900lbs. And the recommended tire pressures are 35 front, 33 rear.

The Yaris sedan, by comparison, comes with P185/60/R15 tires rated at 1102lbs maximum load at 44 psi, has a curb weight of 2350lbs, and the recommended tire pressures are 32 psi all around.

So let's compare. I believe that the weight distribution of the Prius is skewed more toward the front than for the Yaris, due to the 2 electric motors, etc. So for simplicity, let's say that if it had the same weight distribution as the yaris, the recommendation would be 34/34.

2350 lbs/2900 lbs * 1168 lbs/1102 lbs * 34 psi ~= 29 psi

So... for the Yaris to have the same loading factor as chosen for the Prius, the recommendation would have been 29 psi. However, the Prius does come with low rolling resistance tires, whereas the Yaris' Potenza's nominally are not. Consumer Reports (yeah, but it's the only source I have on this) reports that the real world difference between tires with relatively high RR ratings and relatively low RR ratings is about 1%. Controlled testing by Goodyear regarding fuel efficiency and tire pressure show that a 3 psi reduction, on all four tires, from the placard recommendation results in a bout a 1% reduction in fuel economy.

So, if we add 3 psi to make up for our lack of LRR tires, we get the 32 psi that is recommended on the (or at least my) placard.

I think that is an interesting result. And as I'm enjoying the relatively smooth and quiet ride of the Yaris compared to my trusty old Chevy Sprint Metro, I'm not particularly inclined to increase it.

-Steve

P.S. There are a few facts which I still need to verify. But I'm 90% sure that my numbers are accurate regarding the important stuff.

================

Edit:

(Corrections & References.)

Weight distribution for the Yaris sedan is 61/39. And for the Prius, 59/41. OK. So that assumption was wrong. So rather then 32psi/32psi f/r, the equivalent loading for the Yaris would be more like 33psi/31psi f/r. Still, pretty close.

http://tinyurl.com/y8de7yf
http://tinyurl.com/yhw53an

===================

Info on tire pressure vs fuel economy, plus lot's of other interesting stuff: http://tinyurl.com/yljxs7f

===================

Last edited by sbergman27; 03-23-2010 at 10:58 PM.
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Old 03-23-2010, 11:36 PM   #2
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set ours at 35 > 36 .
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Old 03-24-2010, 09:08 AM   #3
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Originally Posted by sbergman27 View Post

So, if we add 3 psi to make up for our lack of LRR tires, we get the 32 psi that is recommended on the (or at least my) placard.

I think that is an interesting result. And as I'm enjoying the relatively smooth and quiet ride of the Yaris compared to my trusty old Chevy Sprint Metro, I'm not particularly inclined to increase it.

-Steve
And if yours came with the Goodyear tires, like mine, it's not only on the Yaris' placard, it says it in small print on the tire's sidewall, to inflate properly to what manufacturers placard recommendation is.
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Old 03-24-2010, 10:29 AM   #4
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it's not only on the Yaris' placard, it says it in small print on the tire's sidewall, to inflate properly to what manufacturers placard recommendation is.
I never put to much stock into what other people's liability lawyers tell me.


@Steve: That's great info, and it's interesting to see how the recommended pressure is linear between the two vehicles. One thing that's worth adding is that for hypermilers lowering the rolling resistance is just as much about increasing coasting distance as it is for making powered driving more efficient.

On a different note, the first folks I ever knew of that were jacking their tire pressure up to 60 PSI was a segment of the 1st generation Prius crowd.
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Old 03-24-2010, 10:45 AM   #5
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I'm running at cool 40mpg average in my Yaris Sedan. With the temps starting to get warmer around here, I will be getting the summer blend for gas... and hopefully start seeing low to mid 40's in MPG.

I also run a bit high in my tire pressure as well. 39-41 currently.

I also took off my winter tires and put on my summer slicks (ok they ain't slicks, but they have around 20% left when it comes to tread and they are great for rolling resistance)

My ScangaugeII has also done a good job in telling me where I am at for every fill up and how I am doing overall.
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Old 03-24-2010, 01:09 PM   #6
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I never put to much stock into what other people's liability lawyers tell me.


@Steve: That's great info, and it's interesting to see how the recommended pressure is linear between the two vehicles. One thing that's worth adding is that for hypermilers lowering the rolling resistance is just as much about increasing coasting distance as it is for making powered driving more efficient.

On a different note, the first folks I ever knew of that were jacking their tire pressure up to 60 PSI was a segment of the 1st generation Prius crowd.
I put zero stock in someone thinking that they've outsmarted Goodyear's and Toyota's engineers.
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Old 03-24-2010, 01:23 PM   #7
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I put zero stock in someone thinking that they've outsmarted Goodyear's and Toyota's engineers.
The 0.3% fuel economy per 1 psi rule of thumb applies to underinflation. One would expect to see diminishing returns as one increases the pressure above the placard values. Plus, I would expect to see a significantly shallower slope (less return per psi change) for the Prius' LRR tires than for regular tires.

Increasing and decreasing pressure is a trade-off, even between the placard pressure and the sidewall pressure. The tire probably won't fail under normal conditions. But impact resistance of the tire *is* reduced. Shock stresses on the wheel *are* increased. (Think "bending" in the case of steel, and "breaking" in the case of alloy.) Shock stresses on suspension components, such as the strut mounts, *is* increased.

It's not the normal driving that is likely to get you. It's that sheer 3 inch transition you hit in the construction zone, where the not yet repaved road meets the newly laid concrete, that you absolutely were not expecting.

I used to run my Sprint at 44 psi... and did sustain wheel and strut mount damage.

And, of course, stress on nerves and dental fillings is increased.

-Steve

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Old 03-24-2010, 01:29 PM   #8
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Quote:
Originally Posted by sbergman27 View Post
The 0.3% fuel economy per 1 psi rule of thumb applies to underinflation. One would expect to see diminishing returns as one increases the pressure above the placard values. Plus, I would expect to see a significantly shallower slope (less return per psi change) for the Prius' LRR tires than for regular tires.

Increasing and decreasing pressure is a trade-off, even between the placard pressure and the sidewall pressure. The tire probably won't fail under normal conditions. But impact resistance of the tire *is* reduced. Shock stresses on the wheel *are* increased. (Think "bending" in the case of steel, and "breaking" in the case of alloy.) Shock stresses on suspension components, such as the strut mounts, *is* increased.

It's not the normal driving that is likely to get you. It's that shear 4 inch transition you hit in the construction zone, where the not yet repaved road meets the newly laid concrete, that you absolutely were not expecting.

I used to run my Sprint at 44 psi... and did sustain wheel and strut mount damage.

And, of course, stress on nerves and dental fillings is increased.

-Steve
True, and what would probably worry me most is being forced to do a panic stop and getting to skid and crash due to the fact that stopping distance would be increased when tires are overinflated to 60psi, and mine didn't come with abs brakes.
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Old 03-24-2010, 01:30 PM   #9
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That's great info,
Although I mainly included it for the tire pressure info, the last link, although a bit dated, has a great section on "Fuel Efficient Lubricants". It looks like the FE difference between 10w40, 10w30, and 5w30 oils might lie in the range of about 1.2% - 2%. And the difference between 5w20 and 5w30 might lie in the range of about 1% - 2%.

So 5w30 -> 5w20 probably doesn't make more than a about a half an mpg difference. 1 mpg at the very outstide.

-Steve
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Old 03-24-2010, 01:40 PM   #10
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and mine didn't come with abs brakes.
That brings up another interesting trade-off when it comes time to buy new tires. Rolling resistance, tread life, and traction form a sort of triangle. If you want more of one, you likely have to give up some of one or both of the others. I suppose that adding money might help. But still.

I don't have ABS either. And I just recently had to travel for many miles on a layer of sheer, naked ice on I40 between Moriarty and Albuquerque. (The snow plow removed all the snow, right down to the solid ice layer.) This is in the Rockies, of course, so 4% and 6% grades were included. I didn't realize it at first, and happened to hit the brakes, just lightly, to slow down a bit... and all 4 wheels immediately locked. (Ye Gads!!!) Road conditions stayed that way for miles and miles.

I'd been assuming that my next set of tires would have the lowest RR value available. But I've reconsidered that. And now I'm wondering if it is at all possible to *retrofit* ABS.

-Steve
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Old 03-24-2010, 03:21 PM   #11
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achieving 40 + miles per gallon with tires set at 35 > 36 p.s.i.. Not worth premature wear of suspension .
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Old 04-04-2010, 09:04 PM   #12
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achieving 40 + miles per gallon with tires set at 35 > 36 p.s.i.. Not worth premature wear of suspension .
35-36 psi is nothing. That's not going to cause premature wear. 45+ may start to. Some (like Bailout) are running 60 psi. That is going to cause some negative issues but he is aware of that.

The common 32 recommendation is designed as a balance for all considerations.
Ride, handling, efficiency, safety, wear, ect. Different pressures create different negatives and positives for each one individually.
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Old 04-04-2010, 10:33 PM   #13
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Running mine on Nitrogen at 36psi at the recommendation of the specialist at my local tyre place.
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Old 04-04-2010, 11:50 PM   #14
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35-36 psi is nothing. That's not going to cause premature wear. 45+ may start to. Some (like Bailout) are running 60 psi. That is going to cause some negative issues but he is aware of that.

The common 32 recommendation is designed as a balance for all considerations.
Ride, handling, efficiency, safety, wear, ect. Different pressures create different negatives and positives for each one individually.
there is a huge difference between 35 and 40 impact-wise, you don't have
to go all the way to 45 to change the suspension game entirely
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Old 04-05-2010, 12:02 PM   #15
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dont know how much suspension is effected... i'm not going to try and talk sense at anybody when I have no scientific data.... What I noticed the most.... is that with my sound deadened car... somewhere around 40-44psi I notice the ride isn't as smooth as compared to 32psi, and at 49psi I notice my car is rattling more than normal and again isn't as smooth/comfortable of a ride.
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Old 04-05-2010, 02:21 PM   #16
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For the 2007 yaris, its 33 that they say to use from factory. I use 35 all around.
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Old 04-05-2010, 05:14 PM   #17
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Running mine on Nitrogen at 36psi at the recommendation of the specialist at my local tyre place.
Note that the air you are breathing is 78% nitrogen. Running pure nitrogen, your tires might be argon deficient, though. Only way to know for sure is to do a UTA on at least one of them. ;-)

-Steve

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Old 04-05-2010, 05:25 PM   #18
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35-36 psi is nothing. That's not going to cause premature wear. 45+ may start to.
It's not so much wear that I'm concerned about. It's impact damage. Peak impact forces increase dramatically with tire pressure. A strut mount is going to last pretty much forever. It's not going to wear. It can, however, *tear* if subjected to a high enough peak force from impact. The tires are an integral part of the suspension system. Their job entails far more than to just minimize rolling resistance.
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