how high can tyre pressure go?

[mpm235]

Hello,

I like to keep my tyre pressures up (to help fuel consumption).

how high can I practically go on my Gen 2 with 16 inch wheels, and still be practical and not loose traction?

would 39 psi and 37 psi be OK?

many thanks, Mark

[johalareewi]

The tyre will have a maximum sidewall pressure (somewhere on the tyre).

Do not go above this!

In practice, if your tyre pressure is too high, you will wear the center of the tyre faster than the edges.

If your tyre pressure is too low, you will wear the edges of the tyre faster than the center and risk damage to the sidewalls.

So your working pressure range is inbetween.

Different tyres behave differently so you need to experiment carefully and safely and ask around.

 

[Ancient Nerd]

From memories of my old gen 2, the recommended pressures were too low. The edges of the tyres wore significantly more at the edges than in the centre.

I ended up increasing the pressure by several psi to somewhere near the  pressures you are suggesting. Tyre life and mpg improved noticeably , with no apparent change to road holding.  I think it was still on only its fourth set of tyres when part exchanged at 185,000 miles. 

FWIW the same applied to the gen 3 - third set of tyres lightly and evenly worn at 135,000 miles,  after a lifetime at high 30s psi.

Also the  tyres on  my current Auris HSD, which I keep at 38 psi, are  wearing evenly at just over 20,000 miles. 

[Sooty]

I run my tyres at 39psi and 36psi, both 3psi above the higher pressure shown on the door plate.

My tyres are wearing evenly.

HTH.

Sooty

[Joseph D]

4 hours ago, mpm235 said:

would 39 psi and 37 psi be OK?

That is what I run on the tyres I have fitted.

Pay attention to the advice johalareewi gives in Post #2. This is good advice.

Pay particular attention to how the tyre is wearing and adjust according to what you observe of your tyre's wear.

[mpm235]

thanks very much everyone,

cheers, Mark

[MEP's Yaris GS]

I run 38psi all around without issue. Even tyre wear and high milage from them also :)

[Joseph D]

I meant to also mention another point that directly goes to the OP's initial question "how high can you go". There is a diminishing return in fuel economy after about 39/40 PSI and I also believe that as you move into overinflation, as evidenced by tyre wearing in the middle, you are compromising safety.

The assumption I see some are making (in other forums) is that by increasing tyre pressure by a few PSI and seeing improvement in efficiency, increasing a few more must have the same improvement. But, when you understand that the (relatively) huge improvement is because the tyre is no longer being run under-inflated, which I believe the placarded < 60 mph values are, you are seeing a greater benefit than if you increase them into overinflated territory.

Once you start into overinflated territory, as already mentioned, the part of the tyre that contacts the road surface assumes a rounded shape (hence why the tyre wears in the middle) and so there is less of the tyre contacting the road surface which impacts in road handling and more importantly braking efficiency.

I reiterate johalareewi's advice, "...  you need to experiment carefully and safely  ..."

[chi tang]

I find this Topic very interesting. Will up my pressure to suggested and see what happens :)

[Duffryn]

In the unhappy event of being involved in an accident, are there any implications insurance-wise for exceeding the manufacturers recommendations?

[Heidfirst]

yes, they could refuse to pay out.

[Joseph D]

15 hours ago, chi tang said:

I find this Topic very interesting. Will up my pressure to suggested and see what happens :)

I would not "see what happens", just because you read it somewhere. Take some measurements of each of your tyre treads - inside edge, middle and outside edge.  Make adjustments to the tyre pressure, up or down, based on that. You have nothing to gain from over-inflation (or under-inflation, for that matter) just for the sake of it, and probably a lot to lose.

13 hours ago, Heidfirst said:

yes, they could refuse to pay out.

So, if the placarded pressures proved to be under-inflated (evidenced by tyre tread wear) and it was deemed to be the cause of the crash, the insurance company would just payout because you were following the manufacturer's recommendations?  I think not.

Conversely, if you used pressures that proved the tyres to be correctly inflated (evidenced by tyre tread wear) and it was not deemed to be the cause of the crash, the insurance company would not payout just because you were not following the manufacturer's recommendations? Hmmm, if it were me I'd be p1$$ed, and we'd be going to court.

I think the overriding factor would be whether the state of the tyres were deemed to be the cause of the crash.

Let's also remember, the placarded pressure(s) is/are a recommendation, not a hard and fast rule.

[Heidfirst]

2 hours ago, Joseph D said:

So, if the placarded pressures proved to be under-inflated (evidenced by tyre tread wear) and it was deemed to be the cause of the crash, the insurance company would just payout because you were following the manufacturer's recommendations?  I think not.

Conversely, if you used pressures that proved the tyres to be correctly inflated (evidenced by tyre tread wear) and it was not deemed to be the cause of the crash, the insurance company would not payout just because you were not following the manufacturer's recommendations? Hmmm, if it were me I'd be p1$$ed, and we'd be going to court.

Obviously overall tyre condition & any contribution to the causation of an accident would be considered e.g. if somebody runs into you it is unlikely that you are likely to be penalised. Similarly imo they are unlikely to quibble about 2 or 3 psi as it will vary that with temperature anyway.

Unfortunately tyre wear could have been caused by previous over/under-inflation rather than that prevalent at the time of any theoretical accident so I doubt is admissible re tyre pressures - otherwise I could have perfect wear, drop my pressures to 15psi, go out & have an accident & expect no penalty.

Insurance companies can & will get out of paying out based on minor infractions - I read recently that one refused to pay out because a customer had changed his rear number plate bulb to LED.

Your profile says that you are non-UK so perhaps things are different where you are?

[Joseph D]

23 minutes ago, Heidfirst said:

I could have perfect wear, drop my pressures to 15psi, go out & have an accident

In reality people don't do this and if it did really happen, the thought mental illness comes to mind. But I take your point. The reality is both would be considered by any accident investigator with their salt.

23 minutes ago, Heidfirst said:

Insurance companies can & will get out of paying out based on minor infractions

This true, but corporates only get away with this unethical behaviour because they are not held to account. However, it is probably a reflection of the environment they work in. Generally, in insurance, if policyholders do not think twice about committing insurance fraud, you will find the insurance companies will respond in kind.

[PeteB]

More worrying for me is that after a serious accident that warrants the attention of a Police accident investigator, any evidence that tyres were incorrectly inflated or otherwise faulty could not only result in blame being apportioned differently but possibly prosecution as well.

I belong to a RoSPA (Royal Society for the Prevention of Accidents) group and my group's meeting was addressed by such an investigator some time ago.  He did mention that after a KSI (killed/seriously injured) collision, tyre pressures and evidence for the condition of tyres were part of the investigation, including expert scientific analysis where warranted.

I generally keep mine at a maximum of 1-2 psi above that book value, which I believe is a justifiable deviation, and note from my tyre monitoring system that during a long high speed drive on a sunny (not necessarily hot) day tyre temperatures can climb significantly and so can the pressures.

[MEP's Yaris GS]

On 27 May 2016 at 5:11 PM, Heidfirst said:

yes, they could refuse to pay out.

Not quite as true as you seem to think.

There are various factors, long with them having to prove that you've knowingly done something damgerous etc.

In addition to this, there will be various figures, depending upon if you're going by the vehicle manufacture recommendations, or that of the tyre manufacture etc.

Insurance companies can't just repudiate claims left right and centre as some seem to think, and they are heavily accountable both within law and with the FCA.

[Heidfirst]

did you read the following posts? The first post was a brief post because at the time I didn't want to have to post a long post explaining how/why they could potentially avoid paying out ... & no, they don't have to prove that you knowingly did something wrong - ignorance is no defence. 

[barrycoll]

The cycle  world is strongly coming around to running lower pressures for (counter-intuitively) more forward momentum.....so Alan's second post sums up the situation

Essentially, the tyre provides suspension, and any suspension is primarily intended to isolate the bike/car from surface irregularities, other wise it would move

vertically over each bump. The energy required to do this is taken from forward motion, and is not fully regained at the vehicle drops back down, on the other side of

the bump. Of course the motor vehicles also have their own suspension (as do mountain bikes), that absorb some deflection at the expense of forward momentum.

So there is a trade off between suspension loses, and tyre deflection loses. An increase in inflation pressures reduces tyre flex, and therefore loses due to tyre 

internal friction (or hysteresis). Lowering pressures allows the tyre to absorb bumps more effectively, and so reduces suspension loses.

So these two effects cancel each other out, as tyre friction loses mount with reduced pressure, suspension loses fall, and vice versa.

One imagines that Toyota have crunched these numbers (plus multiple other aspects) through their Panasonics and Sonys, and come up with the numbers on the sidewall, as per Alan's second post, above.

 

[Joseph D]

13 hours ago, barrycoll said:

...  The cycle world is strongly coming around to running lower pressures for (counter-intuitively) more forward momentum.....so Alan's second post sums up the situation  ...

Although I kind of take your point to a small degree, comparing vehicle tyres to cycle tyres is like comparing apples to oranges. I feel I can knowledgeably comment, as a cyclist myself, that there are a totally whole different set of criteria involved in cycle tyre pressure compared to vehicle tyre pressure. I'm not even really sure there is a cross over, the design and physics of each is totally different.

13 hours ago, barrycoll said:

One imagines that Toyota have crunched these numbers (plus multiple other aspects) through their Panasonics and Sonys, and come up with the numbers on the sidewall

I doubt very much that Toyota have any input into a tyre manufacturer's max sidewall pressure that is stamped on the tyre.

The only correct pressure is the one where it is neither under-inflated nor over-inflated and the only way to work that out is to examine the tyre tread wear pattern on a regular basis. Like I said previously, the placarded pressures are a recommended starting point, as it can in no way cover all makes and models of tyre.

Also, remember that the internal pressure of tyre changes with the weather/temperature and season, so regular checking and adjustment, by the prudent car owner/driver, is necessary to keep the tyres optimally aired.

[barrycoll]

Yes, you are right Joseph, as the sidewall stamp is the tyre manufacturer's  figures and not Toy's.

And as you say, the motor car is far more complicated to the power of 10, over  a push bike, but we are still back to More pressure, or Less pressure as the 

specific parameter that will effect leg power/fuel efficiency. It would make (emotional) sense for More is Better, but discomfort on a bike and excessive tyre wear on the vehicle is the possible end result (all other factors being equal), and with no guarantee of more efficiency.

Mr T. must have configured tyre pressures as part of their need for top-$ economy figures on that new car windscreen.

On the other hand their cars are also bought by 12 heavily armed, bearded blokes, in the back of a Mr T pickup, where tyre pressures are possibly not a top issue

..................But just maybe it is all far more complicated than this bear with a small  brain can accomodate!

[MEP's Yaris GS]

On 29 May 2016 at 2:39 PM, Heidfirst said:

& no, they don't have to prove that you knowingly did something wrong - ignorance is no defence. 

You seem to be getting confused with criminal matters (where ignorance is no defence) and civil matters, which insurance falls under. Insurance is all about negligence and insured perils.

[johalareewi]

On 01/06/2016 at 4:25 AM, Joseph D said:

Also, remember that the internal pressure of tyre changes with the weather/temperature and season

Tyre composition plays a part as well.  Which is why LRR (low rolling resistance) tyres have higher recommended pressures than 'equivalent' non-LRR tyres.  The figures from MrT can only give a ball park / average / good starting point for tyre pressures.

In the hybrid Yaris my wife has, there are two sets of recommended pressures.  One for non-LRR tyres and one (ECO) for LRR tyres.  Trouble is, MrT service dept (around here anyways) always use the non-LRR values even though the tyres are LRR.  Result = under inflated tyres and rubbish mpg.

 

[mpm235]

I have set my pressures at 39 and 37, (they were at 36 and 35, so only gone up a bit).

 I've checked the fuel consumption over the last 400 miles, its gone up from around 53ish to 56ish. So happy days (although the other factors like weather and so on may have played their part).

cheers, Mark

[Cyker]

Yeah, it's hard to be sure with the rising temps also improving mpg, but I experienced similar gains when I went from 35 to 38 (front) and 30 to 35 (rear).

It also fixed the excessive shoulder wear I was getting to boot!

I think Toyota tend to err on the lower side as the ride is much softer and quieter. The increased tyre pressures on mine are making my already noisy Dunlops annoyingly loud on the rougher parts of the motorway and A-roads!

You have to be careful with LRR's tho' - REAL LRRs should not need higher PSI than normal, and in fact I found it dangerous as the harder sidewalls and tyre compound tolerated increased PSI's far worse than normal tyres, and had noticeably worse grip.

Increasing PSI on a normal tyre is a poor-man's way of getting LRR benefits out of a normal tyre; If you already have a LRR you shouldn't need to do this, but there are a lot of tyres being marketed as LRR these days when they're really not.

 

[PeteB]

My previous 2012 Prius predated the fitting of Tyre Pressure Monitoring systems, so I installed an aftermarket system that displays temperatures as well as individual tyre pressures.  It gave me quite a few surprises.  The biggest influence on tyre pressures is by far sunlight, even weak sunlight.

What really surprised me was the side in stronger sunlight can reach 5-10°C above the side in the shade, and the hotter tyres can display a pressure of 3-5 psi above those on the shaded side, which themselves may have risen by 1-2 PSI.

Driving at 70 mph for any length of time seems to only add 2-3°C and 2-3 PSI to the pressure (with 2 people and lots of luggage) - that's on top of any increase caused by sunlight falling on the tyres.

So potentially, up to 10 PSI increase at speed on a hot day, which is another reason I avoid the temptation to over inflate to boost mpg.