Tyre pressure testing

[Funkyboy]

Hi All,

When is the best time to test tyre pressure?
My car is fitted with a tyre pressure monitoring system. Tyres should be 35psi. If they are set to that on a cold tyre after driving they read at a much higher pressure obviously because tyres have warmed up.
So when should you take a reading?

Thanks

 

[DRIVER200]

Hi All,

When is the best time to test tyre pressure?
My car is fitted with a tyre pressure monitoring system. Tyres should be 35psi. If they are set to that on a cold tyre after driving they read at a much higher pressure obviously because tyres have warmed up.
So when should you take a reading?

Thanks

Well you sort of answered your own question check them when cold, say first thing in the morning.
You can double check their pressure through SERVICE after you have been driving for a few minutes which is actually quite accurate. But obviously will increase the warmer they get. You can also use MERCEDES COMAND ON-LINE which us reasonably accurate but don't rely on it.

 

[Codger49]

Think you will find all tyre pressure values are given for cold tyres.

Doesn't seem to be any allowances made for air temperature range though.

Perhaps our 0 to 25 range isn't sufficient to affect tyres?

 

[190]

It's more than sufficient to affect tyres. Pressures change by approx. 4 PSI over that temperature range.

You need to set them cold at whatever the ambient temperature happens to be. If set cold in the depths of winter and assuming no leaks they would need some air letting out as the weather gets warmer.

 

[whitenemesis]

Very roughly speaking tyre pressures increase/decrease 1psi with every 10 degF change in ambient temp.

When manufacturers set the cold inflation pressure they take into account the increase in pressure due to driving, the flexing of tread and sidewall generating heat. In service the pressure will be at the design pressure. This is often why one might have different pressure ranges for laden and unladen vehicles, or normal and high speed driving.

Always measure tyre pressures first thing in the morning, before driving.

 

[190]

Fundamentally it's the hot pressure that counts but it would be too inconsistent for the general public to set them hot. Race teams worry about the hot pressure/temperature and set the cold pressure in order to meet their hot targets.

In a similar way there is a rule of thumb with motorcycle tyre pressures where if you measure them cold the pressure should rise by 10% after a spirited ride. If they rise more then the cold pressure was too low and excessive tyre flexing caused a greater temperature increase. If they don't rise enough them the cold pressure was too high and not enough flexing is taking place.