Spare Tyre Pressure

It is a tiny space saver tire, designed to limp to the nearest repair facility. It is not a real tire you can drive at highway speeds on. It is cheap and light, however.

Spare tire - Wikipedia, the free encyclopedia

 

Smaller air volume = higher pressure. My skinny tired racing bike tires are at 120 psi, but a fat tired mountain bike tire is much lower.

SCH-I535

 

Pablo808 – Greetings from “Up Under.” There are likely a couple of reasons to explain the 60 PSI pressure for the Spare Tire called out in the Owner’s Manual. First, as JimboPalmer says, “It is a tiny space saver tire, designed to limp to the nearest repair facility. It is not a real tire you can drive at highway speeds on. It is cheap and light…” The 33/32 “Cold” PSI that Toyota specifies for the four regular tires is selected with considerations for trading off ride-comfort-handling. The 60 PSI for the Space Saver Spare is all about “Emergency-Temporary Use, Limping Along” a short distance to have the regular tire repaired/replaced, not safe, long term use or ride-comfort-handling.

The high 60 PSI called out for the Space Saver Spare may be because it is designed-constructed as a “Cheapo Tire,” and the 60 PSI aids it in running “Cooler” to keep it from heating up too high and failing when used, along with possible unintended, ancillary benefits for slow pressure leakage over time, between pressure checks, as it sits in the rear well waiting for emergency use.

FWIW FYI: I have a Prius v(vee) Five. In the interest of maximizing MPG-Fuel Economy, I run the regular 17-inch, P215/50R17, low-profile tires on my Prius v(vee) Up-Pressured to the tire manufacture’s Maximum “Cold Pressure” PSI number embossed on the side wall, and maintain the 1 PSI Front/Rear difference (i.e. 51 PSI Front/50 PSI Rear). This PSI level Up-Pressuring delivers around 2 MPGs in additional incremental fuel economy to me, as well as extending the tread life of the P215/50R17s.

 

Like mentioned, the smaller volume of air needs to help support the weight/load. The lesser volume of the spare means it requires more pressure to support the load. It should be at 60 psi when used, and is not that high because by the time you use it it is a lot less.

 

Smaller tire footprint, higher tire loading, requiring higher internal pressure to support.

 

Even with the higher psi these donut tires are not safe. I had a flat in NJ and the donut (on the rear, never on the front) was destroyed in less than a mile by a pot hole at 30 mph (the rim was bent releasing the air pressure) which left me stranded on mothers day 100 miles from home.

JeffD

 

Jeff, I'd wager that your spare had not had its pressure checked in years. Air leaks through the rubber over time and few service facilities really do a thorough job of checking all 5 pressures and some clowns even set them all the same...easy for them.

 

The pressure was checked when the donut was put on the rear. The rim is very soft steel.

JeffD

 

The higher pressure is required because of the width of the emergency spare. When placed on the car, a tire deforms until the total force of the road on the tire surface equals the total force in opposition over that same contact patch. For the same tire, the lower the pressure the bigger the contact patch and greater the deformation needed to achieve it. Similarly, for a narrower tire a higher pressure is needed to achieve the same amount of deformation as in a wider one. In other words, to have a similar lowering of the rim to the ground due to the weight of the car on your narrow spare in comparison to your normal tire you need a higher pressure.

Why this matters is that if you run too low a pressure you run the risk of impacting your rim on a pothole, even at 30 mph, which is going to cause you a problem (see above post). Much like on a road bike (which I ride these days about the same mileage as my Prius v) you want to keep your tires low enough in pressure as to avoid unnecessary risk of debris punctures (glass, nails, or even sharp stones in the case of a bike) and high enough to avoid "pinch" flats, where the tire is pinched between the road and the rim when nailing a pothole.

One thing you can do is to take advantage of the rather large imbalance in the weight distribution of the Prius (and most fwd cars) It is about 60/40 f/r, so you may want to consider a impromptu tire rotation if you get a front flat and are uncertain about the tire pressure in the spare, or if you know it is less than the recommended psi.

 

yup.

 

Pablo808 – Yes, the ride is "much harder" at the Up-Pressured 51 PSI Front/50 PSI Rear levels, and you can feel the road more. The 33 PSI Front/32 PSI Rear on the door post does give you a softer ride. The added Up-Pressuring also gives you more margin against damage to the 17-inch wheels, should you inadvertently hit a pot hole or experience a similar impact to the wheels from the road while driving, than does running the low-profile (shortened side wall dimension) P215/50R17s at the OEM recommended/specified 33 PSI Front/32 PSI Rear.

As regards fuel economy and Up-Pressuring tire pressure, the U.S. EPA rule-of-thumb for tire inflation is that every 1-PSI increase in "Cold" tire air pressure will deliver an incremental gain in fuel economy of from 0.3 to 0.4 MPG.

 

Your EPA "Rule-of-Thumb" makes several assumptions. The key ones for Prii owners are:

• An average car - 2o-25 MPG and the savings are mis-stated for us with an average MPG closer to 50
• The savings are NOT linear. You get more improvement with the early increase in PSI and then get diminishing returns as you increase the psi further.

I run 42/40 and have done that for 11 years - most recently with Nokian WRg2 tires.

JeffD

 

JDEenenberg “JeffD” – The point of the “EPA’s Rule Of Thumb” is that Up-Pressured tires add to fuel economy, compared to the same tires-wheels whose air pressure is set at lower PSI levels. Yes, the MPG savings are neither a precise prediction nor are they linear, particularly, for those of us running at “Stellar” levels of MPG.

A prime example is my current Prius v(vee), which I run with its P215/50R17s Low-Profile tires inflated at 18 PSI above the OEM “Door Post” PSI numbers. Using EPA’s “Rule-Of-Thumb,” the 18-PSI Up-Pressuring would predict that 5.4-To-7.2 MPGs of added fuel economy be delivered to me. However, in practice, I very conservatively see about an incremental 2+MPG’ish addition to fuel economy, with the possibility that the 18 PSI Up-Pressuring may produce an incremental bonus MPG fuel economy boost over the range 2.0-To-4.0 MPGs running on those Toyo Proxes A20 P215/50R17s.

 

I'm trying to understand where you get a 5.4 to 7.2 increase in mpg. The EPA website on gas mileage tips says:

I've never found anything on any EPA website regarding up-pressuring (which I do) and I don't think I've ever heard it asserting anything like this. Just curious.

 

NSFbr - "Up-Pressuring" is a term I coined, and use. It is not the EPA's term. I extracted the "0.3-To-0.4...Rule-Of-Thumb" from the Official EPA site, over seven-years ago. I cannot comment on how this original EPA "0.3-To-0.4...Rule-Of-Thumb" blurb may or may not have been tweaked, changed, adjusted, or reworded and reworked since then.

I am just sharing with the gathered eyeballs what I do, and reporting the results I obtain from doing them. Readers are free to take my opinion, and implement my practices and techniques, or ignore them. If you or others do not believe the EPA "0.3-To-0.4...Rule-Of-Thumb," then ignore it, and run the OEM P215/50R17s tires on the Prius v (vee) pressured at Toyota's recommended 33 PSI Front/32 PSI Rear shown on the door post placard. As for me, I shall continue chasing every easy to get incremental MPG I can get. I shall be continuing to run my OEM Toyo Proxes A20 P215/50R17s tires at 51 PSI Front/50 PSI Rear.

FWIW FYI: On my old, iconic GEN II/2G, "Up-Pressuring" the tires rewarded me with an incremental extra 3-ish MPG Per Tank-Full, over what I was logging running my 2G Petrol Warfighter on the 2G's OEM Goodyear LRR tires pressured at the "Door Post" PSIs. As I reported above, "Up-Pressuring" my OEM Toyo Proxes A20 P215/50R17s tires has delivered me an incremental extra 2+ MPGs.

 

Sorry for the long reply. I'm wordy. TL;DR summary: Diminishing, and eventually negative returns from increasing tire pressure too much. Door jam numbers are absurdly low. Best numbers somewhat lower than you seem to be running at, but much higher than the door jam.


Okay, so here is where I think - my opinion, but based on some very real experience and access to test data (albeit ~15 years old) from a major tire manufacturer from Japan - you go wrong. The EPA's rule of thumb speaks to underinflation, and very specifically does not speak to inflating tires above the recommended level. This really matters because rolling resistance is comprised of two main elements - sidewall flex and tread flex. There are other factors, but those are the 95% constituents.

Underinflating your tires greatly increases losses due to sidewall flex. Greatly. To the point where you can ruin the tires if you do it long enough. That is what you are fixing when you pump up the tires. But, after a certain point, there is no more to gain from doing so. Yes, you can start to reduce the already small loss of the contact patch flex itself, but that is small anyway. (This ignores the benefit of overinflating tires if you have alignment issues. Much better to get the car aligned though.) So the EPA is talking about the increased loss from having a tire with excessive sidewall flex. Get rid of that excessive flex, get rid of the increased loss.

Now here is the flip side - as you pump your tires up more and more, you actually begin to increase the losses due to tread flex. That is, the squirming around of the treads into the crevices in the road and the open gaps in the tire. Keep in mind that tire rubber is incompressible, so when the tires are pumped up more, the parts of the tire that actually contact the road must deform more. Higher pressure, more deformation. more deformation, more wear as compared to proper inflation when traveling in a straight light. By the way, this is the reason that slicks, or worn tires for that matter, have lower rolling resistance. Less squirming.

Slightly counter intuitive, I know. But this is also why you can get very different mileage depending on the road surface. Nice smooth black top with minimal surface roughness - go ahead pump up to 50 psi. It is probably still as good as you can get, or at worst, not any worse than down at say 40 psi. But when you are on the more common slightly open asphalt used to improve porousity to avoid standing water, I promise, 50 psi is not your friend.

The last big negative about overinflating your tires is that it increases the losses due to the unsprung mass of the car. Again, that is road dependent, so on a perfectly flat, smooth highway it makes little or no difference. On a less than perfect road, it can matter quite a bit.

If I still have you let me clarify that by the terms underinflation and overinflation I refer to pressures on either side of optimal inflation. And that bears no relation to what is on the door jam of the car. That value is absurdly low (a point of agreement I expect!) My best guess as to what is optimal is that it is in the low 40s for the front and about 40 for the rear on the v. The purely empirical reason for saying that is that I saw over a series of drive tests, both on the v and on my previous gen II, that the increase in mpg stopped being detectable about 40/38, and my knowledge about how the curves should behave tells me that there is still a little bit to gain above that point (just smaller than I can measure over the course of a controlled drive test.) So, I pump to 43/41, and top up when I see something below 42/40 in any location.

So yes, I have no intention of ever running at what is indicated on the door jam. There are many reasons for those numbers. Maximizing mpg isn't one of them.