what is the psi in the dunlop direzza 215/50/17. the side of the tire says 51 max psi?

The only inflation pressure RECOMMENDATION on the sidewall of the tire is written in fine print where it says;

"SAFETY WARNING: Serious injury may result from tire failure due to underinflation/overloading - FOLLOW OWNERS MANUAL OR TIRE PLACARD IN VEHICLE...(emphasis added)"

So, look in your owners manual or on the Tire Load and Inflation tag inside the drivers door frame for the correct inflation pressure for that tire on YOUR vehicle, which apparently is 32 front, 29 rear.

By the way, all Standard Load Range passenger tires say "Maxium Inflation 44 psi" on the sidewall and all Extra Load Range (sometimes called Reinforced) tires say "Maximum Inflation 51 psi) on the sidewall. It's an industry standard. But like it says, that is a MAXIMUM inflation pressure the tire is designed to take without killing you, not a RECOMMENDATION. It does not matter if you replaced your original SL range tires with XL range tires, you still inflate them the same on your car - 32/29.

I'll bet the tires with the 51 max psi rating didn't come on the car when new. You forgot to tell us the year, make and model of your car. How are we to know what the correct tire and wheel size should be on the car? Car manufacturers want the average 75 year old driver to feel like their back-sides are loaded with Novocaine. If your car didn't come with 51 psi max pressure tires, split the difference between 32 and 51 and see if you can take a stiffer riding tire. Keep an eye on tire wear particularly if you have narrow wheels. The tires will wear faster in the center if they have higher pressures and narrow wheels. Every tire on each car I own has an maximum inflation pressure of 51 psi.

Dunlop Direzza Tires

Go with the psi stamped on the tire. The manufacturer decides this based on engineering tests. Tires generally are taking more pressure now than say, 20 or even 10 years ago. Less rolling resistance. Those numbers inside your car door are only good with the original equipment tires...I'm sure they weren't the same type of performance rubber! BTW, you don't absolutely HAVE to inflate to the maximum allowable pressure. A little less pressure (say, 3-4 lbs) can smooth out the ride a bit and it tends to flatten out the tire a little (which MAY improve braking and cornering ).

No matter what brand of tire you get you should always go by the car manufacture Label

You never go by the tire ... you go by the manufacturer label of the car. Only put in what is recommended by manufacturer

Thats the recommended safety level for pounds per square inch. you wouldnt want to go to that as you would be getting a bumpy ride. Your handbook is a guide but once you start changing to different tyres it can differ. I use 35 front and back but it is not recommended .

pounds per square inch = air pressure