Did I miss where you said the mfg, model, and size of the tire ? with 75mph max, it's probably not an ST tire, but could be...
do a google search for the specs on your tires and it will give you an inflation chart to show you the load carrying at different PSI's.
but, putting 10 psi more than the suggested psi is not a big deal, in fact, some tire mfg say that is what you should do to allow faster speeds and more carrying capacity(to a point
)
besides, gauge accuracy, outside temp, road temp, sunshine on the tires, etc... will change the psi just sitting still...
IMHO (and most tire guys), too much psi is MUCH better than not enough PSI !