On tire pressures, the Yaris, and the Prius - Toyota Yaris Forums

sbergman27 · 03-23-2010, 09:25 PM

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.

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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

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

03-23-2010, 09:25 PM	#1
sbergman27 Drives: 2008 Yaris Sedan Join Date: Feb 2010 Location: Oklahoma City, OK Posts: 323	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.