What tire pressure should I run?

[JockoT]

Originally Posted by Gasoline Fumes

Go ahead and experiment! I have 55-60 PSI in my junkyard tires.

I'm pretty sure I have more grip than the smaller original size tires at correct pressures.

If you have cheap junkyard tyres then go ahead and experiment. Most of us have paid through the nose for our tyres and don't want to wear them out sooner than necessary.

If you fit wide tyres at a higher pressure you probably will have better grip than narrow tyres at recommended pressures. But wide wheels steal mpg and I thought that increased mpg was the whole point of the exercise.

[ksa8907]

If you want to argue contact patch size and traction, you need to run the physics equations for friction.

There will NEVER be a big name organization that will contradict the tire and automobile manufacturers recommendations for tire inflation. Why? Liability in the event of a fatal crash. Even if the tire inflation would make no difference, there could be speculation and a sympathetic jury with a wrongful death/gross negligence lawsuit.

[oldtamiyaphile]

Darin ran a bunch of coast down tests are different pressures. The result was that there was a definite point of diminishing return at around 45psi, although that was with a light weight Metro. My Renault tyres are rated for 80PSI - I run them at ~65, my rule being to run as much pressure as my vertebrae will allow.

Mechanics always let my tyres down

[oldtamiyaphile]

Originally Posted by JockoT

If you have cheap junkyard tyres then go ahead and experiment. Most of us have paid through the nose for our tyres and don't want to wear them out sooner than necessary.

I always buy premium tyres (Contis or Michelins), but over the life of a van tyre I will use about $10,000 worth of fuel. Sacrificing a bit of tyre life ~$800/set can still make financial sense.

[JockoT]

Originally Posted by oldtamiyaphile

I always buy premium tyres (Contis or Michelins), but over the life of a van tyre I will use about $10,000 worth of fuel. Sacrificing a bit of tyre life ~$800/set can still make financial sense.

That is certainly the financial way to look at it.
Over the life of a set of tyres, I would have to make a fuel saving of 8% (55 to 59 mpg) to break even. Would I see that sort of improvement in my fuel usage by running at elevated pressures?
Mind you, any appreciable deterioration in tyre life would make even 8% uneconomical for me.

[ksa8907]

Originally Posted by JockoT

That is certainly the financial way to look at it.
Over the life of a set of tyres, I would have to make a fuel saving of 8% (55 to 59 mpg) to break even. Would I see that sort of improvement in my fuel usage by running at elevated pressures?
Mind you, any appreciable deterioration in tyre life would make even 8% uneconomical for me.

Higher inflation pressures reduce wear by reducing deflection of the material and thus reducing frictional heating and degredation.

The downsides to higher inflation pressures are a harsher ride, obviously, and slightly less ulttimate grip.

[EcoCivic]

Originally Posted by oldtamiyaphile

Darin ran a bunch of coast down tests are different pressures. The result was that there was a definite point of diminishing return at around 45psi, although that was with a light weight Metro. My Renault tyres are rated for 80PSI - I run them at ~65, my rule being to run as much pressure as my vertebrae will allow.

Mechanics always let my tyres down

My tires are only rated for 44 PSI maximum, so I do not want to go over 40 PSI because of the pressure increase as the tire heats up, and if I do, the pressure on a hot summer day of prolonged driving could easily increase above 44 PSI, and I do not want to cause a blowout or damage to the tires by exceeding the maximum safe pressure.

[Frank Lee]

Undamaged tires won't blow until there's about 200 psi.

[JRMichler]

The rated pressure is the cold pressure. The hot pressure is what it is.

Tires get hot from hysteresis loss caused by the rubber flexing. More pressure makes the rubber flex less, so the hot pressure is closer to the cold pressure.

It works the other way when the tire pressure is too low. That's why underinflated tires blow out.

[Jez77]

I'd personally never sacrifice grip to save a little bit of fuel especially in the rainy season (although in theory higher pressure is less likely to hydroplane ).

I wonder if the tables get turned on rougher roads?
I know with mtb bike tyres lower pressure means lower resistance on dirt trails within reason.
Once you get away from the citys the roads get pretty rough in OZ