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What Is Correct Tyre Pressure?


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#1 Shibs.

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Posted 25 April 2010 - 03:44 PM

Noob question. Just got new wheels and tyres for my car. I've got 16" Kumho Esta KH11 tyres. How do I know what is the correct tyre pressure for that?

Sorry if the question sounds stupid. This is my first car after all. lol

#2 Dont_tell_him_pike

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Posted 25 April 2010 - 03:55 PM

The folowing guide allows you to work out how the standard pressure (1.8 bar) changes depending on wheel and tyre size:

http://www.omicron.u...?showtopic=5391

However it's worth noting that a few people have suggested it's best to inflate more than standard to improve fuel economy. Can't remember what that thread was called though...
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#3 Shibs.

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Posted 25 April 2010 - 04:02 PM

Holy cow! All the maths gonna do me in. lol I was thinking of a simple answer like 32 or 34psi or something...

I check the sticker on the door and it said 32psi for 15, 16 and 17" wheels...


Also another problem. I notice bigger wheels also mean inaccurate speed readings on the dials. Wonder if the cops will buy that excuse. How can I fix that?

#4 Graham

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Posted 26 April 2010 - 12:26 PM

I used 36 psi front and 34 psi rear and found it to be best for handling.

It was recommended to me during an advanced driver training course that 36psi for the front of a FWD Front Engined car or rear of a mid/rear engined car and 32psi on the other end.
a Front engined RWD car should have 34psi both ends as the weight is generally better distributed.

However, on our other car, it's got 215/60 tyres and even with 36 psi it wears the outside of the tyre too fast, so it's now inflated to 40psi and although the ride comfort is significantly down, the handling is much better.

Other people will have other opinions for various reasons, just make sure you don't exceed any maximum pressures listed on the side of the tyre - there could be different tyre construction techniques that don't like 40psi, but over here where the weather is temporate all year round, I don't think we have anything "exotic" - eg: M&S tyres are almost unheard of.


As for speedo changes when increasing wheel sizes - you're supposed to lower the profile of the tyre when increasing the wheel size so that the overall rolling diameter remains almost the same.
Here in NSW, AUstralia, the regulations allow only a + or - 15mm variance. which is only about a 2.5% difference.

Here's some more maths to work it out - wheel rim (inches) x 25.4 + tyre width * tyre profile / 50.
eg: If the Sera has 185/65/14 tyres stock, that's 14 x 25.4 + 185 * 65 / 50 = 596.1mm
if you go with 16" wheels then 195/50/16 = 601.4mm - the difference is 5.3mm = 0.9% faster. So if your speedo showed 100, you'd be doing almost 101.


D'oh. And now I go off to read the link about calculating pressures to find that it shows the above calculations to work out a tyres rolling diameter!

#5 Shibs.

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Posted 26 April 2010 - 03:18 PM

Thanks. My speedo is currently -5 km/h off the mark. I have a hud speedo so that is not a problem but just wondering if that can be fixed.



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