Some time ago I read an article about the 4 psi rule & thought it might be an interesting topic of discussion.
This is the article I read.
Basically, the 4psi rule looks for an increase in tyre pressure from cold start pressures of 4psi after an hours driving (or thereabouts) If you start with 30psi in the morning, then after an hour of driving the pressure should be 34psi. If it increases MORE than 4psi, then your start pressure was TOO LOW, add more air now; LESS than 4psi then your start pressure was TOO HIGH, drop some pressure now.
A rough guide to adjusting pressures on the go (once the tyres are warm that is) is to adjust your pressure up or down as indicated by the 4psi rule by 1/2 the difference between what the pressure actually IS and what it SHOULD HAVE BEEN if it complied with the 4psi rule. So if you started at 30 psi and it only went up to 32 then the start pressure of 30 was too high, drop 1/2 the difference between 32 (what it IS) and 34 (what it SHOULD BE) ie drop 1 psi NOW!! And tomorrow morning, or the next time you start off on cold tyres, your start pressure should be 29psi not 30. If your pressure doesn't go up ANY at all, then drop 2 psi! Rough guide, but it works.
The only thing about this 4psi rule is that it means you need to make your tyre pressure gauge your friend, and use it a lot for a while. And you need to work out what the optimum pressure is for every load condition, road surface, and ambient temp that you drive in. Sounds like an imposition, but it really doesn't take all that long for most to get on top of it and work out the sorts of variations in start pressure that suit them (and remember, what works for you won't necessarily work for anyone else, even in your car - so it means all those 'what's the right pressure??' questions are meaningless cos it's gonna be a little different for everyone!) With a consistent tyre pressure gauge (consistency is more important than absolute accuracy!) most can get pretty good at estimating what is the correct pressure for any surface/load/temp etc with just a couple of weeks of regular checking - even just checking your tyres in the morning then seeing what pressure they get to when you get to work will help!
Ok now you've had a read, whats your thoughts on the subject?
Cheers
Paul