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Tyre air pressure

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Hi everyone,

Just wondering what everyone pumps their air pressure to?

Nowadays street tyres have pretty stiff sidewalls and does not require that much air pressure so I'm wondering what would be the best between comfort(?) and grip.

Please state your rim size and tyre width(?). Not sure if tyre width affects the tyre pressure but I think it does :p


Just getting a rough idea of what you guys pump to.
 

Nicely

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It varies considerably dependent on size, width and profile. Follow the formula in post 7 of this thread (linked in Useful Threads) and you can't go wrong.

I have 235x40x18 front, 255x35x18 rear and run 29/31.
 
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For those who is lazy to click

ReTeP; said:
Not quite Juz, This is a question thats bugged me ever since fitting fat 16's on my 205 gti so i just worked it out... First Imagine the tyre compresses under weight by a percentge of its wall height and the rims are correct for the width...

For wider rims + tyres (same wall size = same rim diameter) you need lower pressure to get the same effective stiffness (tyre 'squish) hence comfort. This is because the pressure is acting over a larger area - wider rim thus more force.

But for lower profile tyres you want higher pressure to get the same %compression under the cars weight. Possibly more to stop rims getting dented on potholes.

Both are linear relationships (well the wall depth isn't but can be sensibly linearised for small changes) hence for matched tyre+rim sizes (and equal rolling radius) a good approximation would be:

P= tyre pressure
RW= Rim width (inside rim eg. 8 inch) can be subbed for tyre width if you dont have streched/fat look but not ideal
TW= Tyre width (on tyre side eg. 225)
TD= tyre depth (on tyre side eg. 50)
1= original manufacturers spec
2= your new shiny wheels


P2 = P1*(RW1/RW2) -to account for wider rims
P2 = P1*(TW1*TD1)/(TW2*TD2) -to account for smaller tyre wall

Combine these and you get: P2 = P1*(RW1/RW2)*(TW1*TD1)/(TW2*TD2)
Simplified:

P2 = (P1*RW1*TW1*TD1)/(RW2*TD2*TW2)

This works for mixed units as they are fractions and the reason the tyre depth is found from width multiplied by depth is that the depth rating on tyres is actually a percentage of width. A worked example:

S14:
P1 = 32psi, RW = 6.5 inch, TW = 205, TD = 55.

New wheels:
RW = 7.5 inch, TW = 225, TD = 40

P2 = (32*6.5*205*55)/(7.5*40*225) = 37 psi


Only a quick formula i worked out but serves as a pointer. Realistically compressed volume ratios need to be matched but the results wont be far wrong . That means my UK 300zx rims should be run about 28 psi and i always used to run my 205 GTI's tyres too hard - no wonder it was sooo bumpy!!!
 
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Nicely,

My stock ones were 6.5" and 215. Not sure on tyre pressure but I put 32.

my new rims are 8" 235/40 and 9" 225/50.
 

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I get 31/23. based on the standard alloys and pressure. That still seems quite low at the back. Think I'd be inclined to drop it to 23, take a look at what it looks like and put it back up to around 28/9 if it looks flat :p That formula is fairly sound, but that's an interesting result.
 
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38PSI here. Better mileage!!! If one of the best suspension place in Melbourne put it at that, I don't see why I wanna change it. :p
 

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Watch for that bald spot running dead centre round all your tyres...:p I don't know a single tyre place, good or bad, that knows the correct pressure to put in a tyre, especially when its non-standard...
 
M

mitch

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I just bang in 30psi all round regardless of size unless on track, in which case I increase the rear pressure a bit.
 
S

Sic15

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Hi.

With 235/45/17 tyres all round I run 31 front and 29 rear unless car is loaded up with gear for a track day in which case I add 2 lbs to front and rear.

The tyres, Falken RT 615's, are legal for road but also good for the track - I run same presures when tyres are cold - on the track. I check presures after each run on the track ( 4 to 6 laps of a 3.1 or 3.4 km circuit) and adjust to 38 F and 35 R. This works for me.

Cheers
 
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