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Pressure of Tyres?


Danny01
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im on 235/40/r18s and im running 36 on the front and 34 on the back at cold. can't complain about bumps though, with sportivo suspension and fairly low profile tyres - a bit of a bone rattler on rough roads. but i love it :D

getting pretty good even wear on all tyres too

Edited by lateralus
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You can calculate the effect of temperature on your tire pressure using the ideal gas law:

PV = nRT

P = pressure

V = volume

n = no. moles of gas

R = universal gas constant (8.314 J mol-1 K-1)

T = temperature

You want to know the difference in the pressure when you change the temperature, keeping everything else constant:

P2*V n*R*T2

------ = ----------

P1*V n*R*T1

Which simplifies to:

P2 / P1 = T2 / T1

So let's say on a 25 degree day your tires are inflated to 200kPa (29PSI). What's the pressure at 30, 40, 50, 60 and 70 degrees? First, convert the temperature from Celcius to Kelvin by adding 273.

P2 = (T2 / T1) * P1

P2 = [(30 K + 273 K) / (25 K + 273 K)) * P1

Then substitute P1 and evaluate:

P2 = (303 K / 297 K) * 200 kPa

> P2 = 204 kPa (= 29.6 PSI)

40 degrees = 210.8 kPa (= 30.6 PSI)

50 degrees = 217.5 kPa (= 31.5 PSI)

60 degrees = 224.2 kPa (= 32.5 PSI)

70 degrees = 231.0 kPa (= 33.5 PSI)

Edited by Keisari
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You can calculate the effect of temperature on your tire pressure using the ideal gas law:

PV = nRT

P = pressure

V = volume

n = no. moles of gas

R = universal gas constant (8.314 J mol-1 K-1)

T = temperature

You want to know the difference in the pressure when you change the temperature, keeping everything else constant:

P2*V n*R*T2

------ = ----------

P1*V n*R*T1

Which simplifies to:

P2 / P1 = T2 / T1

So let's say on a 25 degree day your tires are inflated to 200kPa (29PSI). What's the pressure at 30, 40, 50, 60 and 70 degrees? First, convert the temperature from Celcius to Kelvin by adding 273.

P2 = (T2 / T1) * P1

P2 = [(30 K + 273 K) / (25 K + 273 K)) * P1

Then substitute P1 and evaluate:

P2 = (303 K / 297 K) * 200 kPa

> P2 = 204 kPa (= 29.6 PSI)

40 degrees = 210.8 kPa (= 30.6 PSI)

50 degrees = 217.5 kPa (= 31.5 PSI)

60 degrees = 224.2 kPa (= 32.5 PSI)

70 degrees = 231.0 kPa (= 33.5 PSI)

lol

nice write up. that will take me a few minutes to work out haha.

when was the last time i did math math.... :huh:

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follow the tyre sticker stuck next to the driver seat?

thats the recommendation for the bog standard wheel and tyre sizes. if you change your size you will need to play around with your pressures. 250kp (front) and 220 kp (rear) for stock wheels wouldnt exactly work well for 19" wheels with 40 aspect ratio tyres.

Edited by lateralus
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im on 235/40/r18s and im running 36 on the front and 34 on the back at cold. can't complain about bumps though, with sportivo suspension and fairly low profile tyres - a bit of a bone rattler on rough roads. but i love it :D

getting pretty good even wear on all tyres too

wow u run 34's at the rears? i wouldve though that would be a tad low.

just wonderin, wot rubber you running?

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sorry, actually meant 35 (not that it would make much difference)

i don't think it's too low. the ride is smooth, with a good compromise between grip and fuel economy. there's no point in having a higher pressure on the rear wheels on a fwd car, as you have no real weight resting on them. so you will just be overinflating them if you were to run 38 on the rears, probably wearing out the centre of your tyres fairly quickly. rwd car, different story.

i've got hankook ventus v8 rs h424's

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sorry, actually meant 35 (not that it would make much difference)

i don't think it's too low. the ride is smooth, with a good compromise between grip and fuel economy. there's no point in having a higher pressure on the rear wheels on a fwd car, as you have no real weight resting on them. so you will just be overinflating them if you were to run 38 on the rears, probably wearing out the centre of your tyres fairly quickly. rwd car, different story.

i've got hankook ventus v8 rs h424's

yea i see wot u mean. however, ive place them on 38rears and done 10,000kms with a travel up 2 falls creek on a full car.

the rears were coping very well with 38's though. dont know why.

when u get new tyres, you see surface with rubber bumps/spikes to verify that it hasnt been used. those bumps/spikes were still there when i rotated them, so mmmmmmmmm

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yea i see wot u mean. however, ive place them on 38rears and done 10,000kms with a travel up 2 falls creek on a full car.

the rears were coping very well with 38's though. dont know why.

when u get new tyres, you see surface with rubber bumps/spikes to verify that it hasnt been used. those bumps/spikes were still there when i rotated them, so mmmmmmmmm

yeah, it also depends on how many people or excess weight youll be carting around and at what speeds. obviously more weight or higher speeds need higher pressures. i mostly drive by myself around town, so i dont really bother changing the pressures to anything higher

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  • 1 month later...

I've got a 1996 Carmy 4 cylinder with Dunlop Daytona 195/70 tyres and usually pump them all up to 35 psi.

Any higher and the ride gets too harsh and choppy.

Going to a lower psi will make the ride more comfortable and soft but have a negative effect on handling and fuel economy.

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A good rule of thumb is 32psi for a passenger vehicle (eg Camry with 15" tyres) and 36psi for a low profile tyre. You can go a bit higher and this will slightly help fuel consumption and the like however it will make the ride a little more hard and bumpy. On passenger tyres you can also go a bit softer for a softer ride however the life of your tyre will decrease and your fuel consumption will go up. I am currently running 36psi on my 225/40R18's and they are going quite nicely.

For 4WD's its a bit harder to say, it will depend on the type of tyres, load etc.

I also work at a tyre shop so there is also a bit of method behind my guide.

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