You need to be careful with tyre pressures - if they aren't inflated to the manufacturers recommended pressures, then you are breaking the law. However, the manufacturer is the tyre manufacturer, not Nissan - if you change brand, you may be able to get the pressure from the manufacturers web site (certainly used to be the case with motorcycle tyres, but if not a call/email to their technical department will give you the answer) and the tyre manufacturer may well have a pressure with an allowance of a couple of psi either way.
Additionally, if you have a serious accident, the police often check tyre pressures to see if that contributed to the cause of the accident - potentially you could find your insurance company arguing with the claim if the pressure is out.
Also, don't forget pressures are measured cold, but a sudden change in temperature can affect the pressure - would have to be a fairly large change though.actd2012-08-01 12:23:31