We have one tyre on the Subaru that lost about 5lb pressure every month last summer, i've done all you suggest John and had the wheel fully submersed, no leaks to be seen, tyre lads can't find it either...this summer it's only lost 2lb per month, i can live with it.

Local tyre chain could not spot a leak. They tried hard. Left me with the bubbles and it took me 1/2 hr to find the non repairable leak right on the edge of the tread. Rather subtle and low leak.

It is not physically possible for a tyre to lose significant pressure and for no bubbles to be visible anywhere - assuming the whole wheel is immersed.

If the seal only breaks when the tyre is under stress.. eg a very slow puncture..it may not show. And if alloy wheel corrosion is the cause, raising the tyre pressure may temporarily seal the tyre to the rim better..

I've heard that tire pressure on the car door is what you want to live by.

I've heard that there is some kind of happy medium between the door listed pressure and the max pressure on the tire.

Nonsense. Just follow the recommendation on the door.

Is there an "add 10psi" or whatever guideline?I'm confused about this from everything I here people say....

Don't take any notice of what 'people' say unless you can be certain of their credentials. I take very little notice of what other people say unless they are emeritus professors of a Russell group university. But many of my posts on here consist of taking issue with posters who propagate nonsense which people without technical knowledge might think is sensible.

I think you're saying to ignore people like you !

<< I take very little notice of what other people say unless they are emeritus professors of a Russell group university. >>

I hope you don't mean that literally when discussing motoring matters, John. My brother-in-law, now retired from an eminent Scottish university, has little clue about his vehicles, making no attempt to monitor their condition and basically watching them deteriorate.

I've heard that tire pressure on the car door is what you want to live by.

I've heard that there is some kind of happy medium between the door listed pressure and the max pressure on the tire.

Nonsense. Just follow the recommendation on the door.

Is there an "add 10psi" or whatever guideline?I'm confused about this from everything I here people say....

Don't take any notice of what 'people' say unless you can be certain of their credentials. I take very little notice of what other people say unless they are emeritus professors of a Russell group university. But many of my posts on here consist of taking issue with posters who propagate nonsense which people without technical knowledge might think is sensible.

I try and go by what makes sense. Obligatory door-score following doesn't (nothing on my door anyway, but recommended pressures are, IIRC, well South of what I use.

Its inevitably going to be a compromise between comfort, handling, fuel economy and tyre wear. If your priorities are different to what the car makers think they should be, a different pressure may suit you better, bearing in mind that you may not have OEM tyres.

I believe you can, if you can be bothered, do a certain amount of ""tuning" with chalk marks on the tyre.Must try it sometime.