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Tyre Pressure at Petrol Station

FZP

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18 Jan 2015
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So the other day I noticed that the handling was feeling a bit custard, so checked pressures and sure enough they were down. According to my gauge they were 31/36. I was expecting it to be off as it's probably been 2 months since I last checked. Warmer weather plays havoc with pressures.
Went to the local petrol station and requested 36/42 as I know they can be a little off. This morning I've retested pressures and I'm still low at 33/38.
Don't trust the machine team, have your own gauge and check them regularly :thumb:
 
Thats cos you warmed your tyres up driving to the fuel station! When they cooled back to ambient the pressures dropped.
 
Tyres should always be checked cold in an ambient environment.

Have you not read your handbook? :wack:
 
Fair point but it's only down the road so don't think it would be enough time to generate any heat. I'm calling dodgy psi gauge at pump
 
Seriously, doesn't take much to get your tyres a few degrees higher. You wanna check the pressure after a hard drive when they're up nearer 50.
 
You're best over inflating on those auto machines to get your money worth. Then using a gauge at home to let the air out to the right pressure :thumb:

All to do with Boyle's law apparently :what: Big Joes (JCX) thread on getting the optimum pressure on his 964 is the stuff of legend :grin:
 
I thought going 2 over would be plenty for the let down this morning. I'll have to go searching for that thread :thumb:
 
Zingari said:
You're best over inflating on those auto machines to get your money worth. Then using a gauge at home to let the air out to the right pressure :thumb:

All to do with Boyle's law apparently :what: Big Joes (JCX) thread on getting the optimum pressure on his 964 is the stuff of legend :grin:
I'm assuming this is the thread? I also found that reading the below thread, I needed to up my pressure to 1.21 jiggawatts at a constant 88mph to ensure I didn't cross the streams....and as we know crossing the streams is very bad.

http://911uk.com/viewtopic.php?t=95813&highlight=tyre*
 
FZP said:
Fair point but it's only down the road so don't think it would be enough time to generate any heat. I'm calling dodgy psi gauge at pump

Dodgy gauge at the pump is generally assumed to be ubiquitous, but beyond that - think about the environment around the rears. Between the engine and the side boxes of the exhaust system on the 997 your rears can actually go up a couple of lbs if the car is just sitting idling for a while. :thumb:
 
Just to add another factor, esp. in summer, if you park outside you can easily get differential heating from the sun - I found it added c.5psi compared to the shady side.
 
After reading JCX thread about pressure, I've inflated to 40/46 and do the let down later tonight and see what happens. I'd not thought about the heat soak effect of both exhaust and engine in and around the rear wheels, but obvious when pointed out. Still, unsure how much heat can be generated in a mile though to make that much difference to psi.
 
Alot.
 
Ok so tyres were set at 40/46 at the same petrol station. This morning and they were 36/41. I've dropped the pressure down to 34/40.
The one thing I can say about Michelin tyres is that they're highly sensitive to pressures. When the pressure is correct they're on another level. When they're off, it makes my car feel like it needs an alignment or something is broken.
Might go find some roundabouts later to test :grin:
 
Never check your coolant level until car has stood overnight and temp is ambient. Same rules of expansion apply :thumb:
 
alex yates said:
Never check your coolant level until car has stood overnight and temp is ambient. Same rules of expansion apply :thumb:

Breaking wind the morning after a mutton tikka vindaloo and 10 pints of Guinness can have the same effect.

I've also tried taking a huge gulp of air before getting on the scales but so far it's not made me any lighter?

Once you get the optimum pressure it might be worth getting them filled with nitrogen to control the heat v pressure change :dont know:
 

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