In reality, the only way to know for sure is to put a "real" gauge on it. The stock ones are not known for their accuracy. I put a good brand name electric one on mine and it was reading low. Thought I had problems, so I put a known good mechanical one on temporarily to double check. Seems the electric one reads 40 psi when it is really 50 psi. Oh well I don't want the oil potentially in the cab so I just take it into account.