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Tyre Pressures


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Can anyone advise, my Rav 4 year 2005, on the door panel states the tyre pressure to be 29 psi, the previous owner changed the wheels from 16 inch to 18 inch, so obviously the tyres are now larger.

I am assuming the tyre pressure should now be increased, does anyone know what pressure I should use, and is their a formula for calculating the pressure.

Thanks for any advice.

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Actually, whenever I've looked, the manufacturer's tyre pressures usually go down when the tyre size is increased.  

I think this is a due to the larger tyre having a bigger contact patch, but only supporting the same weight, so the loading per square inch (cm?) is reduced, and the tyre pressure reduces accordingly.  (I'm assuming the bigger tyre has a shallower sidewall/lower aspect ratio/same rolling radius.)

There are probably better ways, but, you could ring through to, say, the Goodyear Dunlop technical helpline and explain the situation.  

General information: +44 (0)121 378 7000

Technical Helpdesk: +44 (0)8453 453 453

You might have to pretend you've got one of their tyres fitted though.

I'm sure some better ideas will come along shortly.......

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This table is actually for a Toyota Auris, taken from the owners manual.  On this car the differences are non-existant, for the 15", 16" and 17" wheels:-

Over time, checking tyre wear profiles could help you.

e.g. Tyres wearing a greater amount down the tyre centre line - over inflated.  Tyres wearing a greater amount at the edges - underinflated. 

But as you know, other factors can come into play here, such as correct wheel alignment, driving style/pattern.        



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I'd check the manual to see if that's a size listed; If not, you can probably extrapolate an appropriate pressure from the other sizes listed. If all else fails, 35psi is generally a safe place to start!

The Mk4 Yaris has weird pressures - It states, for 15" rims, 41/41psi for Type A and 36/35psi for Type B, whatever the heck Type A and B are, above 100mph (If someone can tell me that the heck Type A and B are in this context I'd be grateful!), and 41/41psi and 33/32 psi for below 100mph

For 16" rims it says 35/32psi for above 100mph and 32/29 psi for below 100mph

For 17" rims it says 32/29psi

I found that weird as I'd have thought lower profiles needed more pressure than high profile tyres, but apparently not??


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