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how high can tyre pressure go?


mpm235
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5 hours ago, PeteB said:

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.

I think (IIRC) the idea of increasing the pressure for high speed driving was to lessen the flexing of the sidewall. The sidewall flexing on an underinflated tyre heats up the tyre and air inside, expanding the air, leading to excess pressure and a blowout. Tyre design and materials are much more advanced these days, so in some cases the tyre (and vehicle) manufacturers don't advocate increasing the tyre pressures. This is shown on the tyre pressure info label on newer cars. E.g. the 06 Gen 2 Prius has different pressures on the label for high and low speed whereas the Gen4 only has one pressure for all. 

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One thing that surprised me about the Gen 4 Prius pressure specs was that cars with 15" wheels and "without a full sized spare wheel" (which I assume means space saver OR gunge) should have 36/35 psi F/R, while 15" and a full sized spare are 32 psi all round. How can that be?

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Highway code:

"Your brakes and steering will be adversely affected by under-inflated or over-inflated tyres"

How much is over-inflation?

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From cold, 10 PSI

...

(I totally made that up)

 

@PeteB - I think they just made that up too! :P


 


 


 

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2 hours ago, PeteB said:

One thing that surprised me about the Gen 4 Prius pressure specs was that cars with 15" wheels and "without a full sized spare wheel" (which I assume means space saver OR gunge) should have 36/35 psi F/R, while 15" and a full sized spare are 32 psi all round. How can that be?

Maybe Toyota fills the spare with helium? :angel:

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