Different tires have different recommended and different max tire pressure ratings. It should be safe to pump up to the max. I don't think it safe to throw out a number like 51psi because not all tires are rated at 51. Mine are rated at 44 max so that is what I run them at. Many tires have a max of 35psi. It is up to the owner to check their own tires ratings and decide for themselves what is safe.