Prius Tyre Pressures

[barrycoll]

Just had my first service, and have discovered that my Halfords tyre pressure guage is under reading...

Having put 32/30 in the tyres I was informed that my tread pattern was down to 5mm on the outside and 7 mms in the centre indicating under inflation....this is after only 6000 miles!

the car has come back with 39/36psi, and feels quite 'lively', shall we say

whether this is to correct the wear pattern, or a faulty dealer tyre guage is a moot point?

should I stick with the 39psi to get more centre wear????

...although it may lead to having more fillings replaced

[ian rooke]

Hi I have found that if you go a couple of pounds over the stated pressures you strike a happy medium for ride and fuel economy.

[Cyker]

Should be okay; You have to go higher than that to get significant centre wear; I used to run my Yaris at 40psi/35psi on Continental PC2E's and if anything it made the wear more even! (At stock recommended pressures I was getting noticeably more shoulder wear!)

It's always best to err on the side of higher pressures than lower - Higher pressures give lower rolling resistance, better road feel and better grip and response at speed but gives a firmer ride. Lower pressures give better grip at low speed and in inclement weather conditions, but reduces cornering stability and at speed is worse in all respects and also causes higher levels of tyre heating which can lead to Surprise! tyre failure at speed.

So if you're not sure, it's better to throw a few extra PSI in than not!

[Joseph D]

… should I stick with the 39psi to get more centre wear???? ...

I would say definitely run a higher pressure for a while until you see the wear even up, although I'd also say you need to run a tad higher pressure than 39. Once you have evened up the wear, drop it to around 38-40F/36-38R - the exact pressure will depend on what you see in regards to the wear pattern of your tyres.

I know each tyre model is different, but I run my tyres at 39F/37R and get even tyre wear. Just use the pressures that get the results you need from the tyres you have fitted.

Also, nice to see a garage that knows something about tyres!

[Sagitar]

I'm running on standard 195/65R15 Ecopias. I have gradually pushed up the pressures to 40 psi front and 38 psi rear to get even wear and may well soon go to 42/40. The PiP is a bit heavier than the standard Prius and I guess that makes some difference.

Of course if you are driving "enthusiastically" and cornering hard all the time, you will get edge wear whatever inflation pressures you choose.

[barrycoll]

well, I must admit to not expecting so many members running tyre pressures even higher than 40psi, which is very re-assuring

it looks like, from Joseph's experience 39/37 creates even wear, so factory pressures are bound to create extra shoulder wear

we are off to Spain at the end of May, onlly 2 up, but complete with bikes, windsurf kit, and all the wife's un-necessary clutter, so would 42/42 be a recommended loaded pressures with a loaded back end??

my style of driving is non-enthusiastic, but always alive and alert to the possibility of squeezing a few more mpg from the car

[Cyker]

There is some conflicting info there; For instance with the Mk1 Yaris we found some owners manuals only list the higher pressures for higher speeds (IIRC 32psi for <90mph and 35psi for >90mph ) but say you don't need to increase pressures for higher loads, whereas my Yaris does list different pressures for both speed and load.

However, if you're running 39/37, I don't think you need to worry too much about boosting them up again for heavier loads as the pressures are already over the manufacturer's recommended. The 37 at the back should be able to handle the extra load just fine.

[johalareewi]

I'm running on standard 195/65R15 Ecopias. I have gradually pushed up the pressures to 40 psi front and 38 psi rear to get even wear and may well soon go to 42/40. The PiP is a bit heavier than the standard Prius and I guess that makes some difference.

The 195/65R15 Michelins on my gen3 prius are at 40/38 and have a nice even wear across the tyre.

[barrycoll]

I am wondering if the stiffer wall of the 17" wheel/tyre combo, would have different wear characteristics to a T3, 15"-er.....

....and possibly benefit from different pressures??

[Cyker]

Could do! I know with bikes, my bro's mountain bike takes something like 60 PSI of pressure for the big chunky off-road tyres, but the ridiculously skinny tyres on his racer need something like 120PSI (!!) to work optimally! (They actually did too; He had to get a new pump because his old one could only get up to 80, but that was apparently too low and was causing high wear on the inner tube!)

[kithmo]

On my Gen 3 T-Spirit I used to up the pressures by 3-4 psi and I swapped the wheels over front to back at 9K miles and at 18K the tyres had 5mm tread left all round and even across the tyre.

[barrycoll]

not sure that bike tires are any help, as the car tire has a less flexible wall, the lower the profile...so 17"'s would have stiffer walls than 15"s at the same pressures....implying that maybe less pressure is necessary?

...or more????

but Keith's experiences with a similar set of 17"s, seems like the right amount of extra pressure, so stop uneven wear

your brother Cyker, must be a heavy chap to be able to pinch the inner tubes while running 80psi

[Duffryn]

Just playing Devil's Advocate for a mo, but does any part of the law cover tyre pressures - over or under - according to manufacturers recommendations? And what about insurance claims - if one was unlucky enough to be involved in an accident and plod discovered one's tyre pressures were way over/under the recommended figures, well I just wonder what an insurance company would make of that?

[kithmo]

I think Mr Plod and Insurance co would only be interested if there was a fatality and 3-4 psi isn't exactly "way over" pressure anyway.

[johalareewi]

Tyre pressues vary as the tyres warm up and cool down so from a physics point of view the actual tyre pressures at the time of the accident are unknown (unless you have TPMS). However, you are right in that these days science and logic don't figure highly in insurance claims and the insurance company will pick up on anything they can to minimise the amount they have to pay out.

Check out Gas Laws (in particular !Removed!-Lussac's law

http://en.wikipedia.org/wiki/Gas_laws

http://en.wikipedia.org/wiki/!Removed!-Lussac's_law

So if you set your tyre pressure to 36PSI in the morning at 7C and it warms up to 17C during the day, your tyre pressures will now be around 38PSI.

[Duffryn]

Blimey, a bit more complex the deeper you go!

However page 551 of my handbook recommends pressures of 33F/32R for 17inchers; over-inflating by c25% as some posters are suggesting might I feel just be a !Removed! of light that the law or insurance companies (when looking to 'blame') might pick up on - if it could be proven..

[Joseph D]

I draw your attention to the word "recommendation". Tyre manufacturers can only recommend an inflation pressure as there are so many variables that go into the "correct pressure". It is really just a starting point. Suffice it to say that the ultimate test of the correct pressure is how the tyre is wearing. The correct pressure is the one where you get even wear across the full width of the tread. It is up to the owner to inspect regularly, be aware of how their tyres are wearing and make adjustments accordingly.

An under-inflated tyre will wear more on the shoulders than the centre of the tyre. Conversely, an over-inflated tyre will wear more in the centre than the shoulders of the tyre. If you are getting wear on only one of the shoulders, it usually means you need your wheel alignment checked and adjusted if necessary.

[Cyker]

your brother Cyker, must be a heavy chap to be able to pinch the inner tubes while running 80psi

He's just big boned!! :D

He's not that heavy really, but the tyres on that bike are razor thin, narrower than my finger! At 80 psi you could still see them flex a bit but at 120 they're practically solid.

I imagine it wasn't very comfortable, cycling through the lovely potholed roads of London with such rock-hard tyres, but it did seem to solve his tube puncture problem!

But the temps thing is a good point; It's best to check/inflate when the tyres are cold, i.e. before a journey. Thankfully, you'd have to inflate a car tyre well over 100psi for it to be in any danger of exploding (Well, assuming the tyre isn't already damaged of course!), so even if you don't it's not the end of the world.

[timberwolf]

I've found with increased tyre pressure, it can have an adverse affect when braking over uneven surfaces such as man-hole covers, it is more likely to "let go" and go floaty as it switches from regenerative to friction braking.

[Sagitar]

I draw your attention to the word "recommendation". Tyre manufacturers can only recommend an inflation pressure as there are so many variables that go into the "correct pressure". It is really just a starting point. Suffice it to say that the ultimate test of the correct pressure is how the tyre is wearing. The correct pressure is the one where you get even wear across the full width of the tread. It is up to the owner to inspect regularly, be aware of how their tyres are wearing and make adjustments accordingly.

An under-inflated tyre will wear more on the shoulders than the centre of the tyre. Conversely, an over-inflated tyre will wear more in the centre than the shoulders of the tyre. If you are getting wear on only one of the shoulders, it usually means you need your wheel alignment checked and adjusted if necessary.

I agree with you in principle Joseph and my personal approach is directly in line with what you advocate.

The fly in the ointment is the Highway Code which says

"Tyres.

Tyres MUST be correctly inflated to the vehicle manufacturer’s specification for the load being carried. Always refer to the vehicle’s handbook or data".

However, the law which underpins this bit of the highway code is Construction and Use Regulation number 27 and it says

"Tyre Pressures

Regulation 27 section b states "the tyre is not so inflated as to make it unfit for the use to which the motor vehicle or trailer is put".

That's pretty ambiguous and there is no indication of the point where an under or over inflated tyre becomes unfit for the use to which it is being put.

The Highway Code requirement is a MUST, so it has the force of law, but the law to which it refers is ambiguous. In using the words " vehicle manufacturer’s specification"

the Code implies that the vehicle manufacturers set a specific level, but as you rightly say, they really only make a recommendation (and realistically can do no more).

There is an upper inflation pressure limit marked on the tyre sidewall, but this is generally much higher than the pressure that you and I would use.

I concluded some time ago that I would be content to argue the case for inflating to the level that gives a sensible wear pattern, but I think there is always a risk that some insurance company might try it on.

[Chris Dance]

I use the cold tyre pressures recommended. When I get the car from a service the pressures are usually higher than recommended usually 5-6 PSI higher. The tyre wear on the Prius Michelins is normal and the car performs OK with regard to braking etc. I bought a new tyre gauge a couple of years ago but you never know if the gauges are 100% acccurate. The inflators at garages are often misused and drivers tend to throw them down. I do not trust garage inflators and gauges.