Tires: tire inflations, psi max, tire inflations

Question
we have a mercedes and in the manual it states to inflate tires according to the placard on the door and inside fuel cap. those placards say to inflate to max 35 psi's however, the tire says max psi 51 lbs. doesn't that mean i can put up to 51 lbs of pressure in the tire? the placard on the inside of the fuel cap for our mercedes says 35 psi max tire inflation and if you go over 100 miles an hour +5psi's.  does that mean if i normally drive over 100 miles per hour you can add 5 more psi's?  my husband says that means your tire will inflate with 5 more psi's because you are driving that fast so you shouldn't put more than 30 psi's if you are driving faster than 100 mph or it will exceed the 35 recommended psi's. this information is all very confusing because i always thought you follow the recommended psi information on the tires.

Answer
Sharon,

What is says on the sidewall is a maximum pressure - that would be the maximum pressure you should use - but it is NOT a recommendation.

What is on the placard IS a specification.  So if you are using the car according the information listed for the 35 psi - you should use 35 psi.

- BUT -

If you are using the car according to what it says for +5 psi, then that is what you should use - and, no, that is NOT what the pressure is to build up to (except coincidentally).

Everything you will see regarding tire pressure will be the "cold" pressure - that is the starting pressure.  You will rarely see any info about pressure buildup - and even then it will be obvious they are talking about that.