First of all Peugeot and Citroen specify along with the pressures tire brand and model: ImageShack - Best place for all of your image hosting and image sharing needs
Now a few things about tire construction I’ve learned from tire company seminars (I operate a tire shop) and from experience. There are two main schools of tire construction.
The first is using a hard, long-lasting tread compound combined with soft sidewalls/carcass. Although hard compounds do not have good traction, the soft carcass permits more tire to come into contact with the road. Most Michelin tires are designed like this.
The other school is using a soft tread compound combined with stiff sidewalls/carcass. Stiff sidewalls have the advantage of making car control more precise, especially at transient situations (S-curves in quick succession). The advantage of these tires is that they have a sportier feel but wear faster. Pirelli PZero Nero is designed like this, other Pirelli tires not so much.
Tires for low-end cars that are marketed as “fuel-saving” have both stiff sidewalls (less rolling resistance) and hard tread compounds (less rolling resistance and long lasting). Needless to say, those tires have terrible traction and unless mileage is the main concern, they should be avoided.
And a real life example: My car comes equipped with Michelin Exalto 2, size 205/50/17. I changed them to Pirelli PZero Nero size 215/45/17 (sidewall height roughly the same with original, about 1cm wider tread) and took the car for a test drive on my favorite mountain pass. I noticed that the sidewall flexed so that part of it actually touched the road surface under heavy cornering. I never had this problem with the Michelins, even on the track.
Here’s a pic. Notice the black line: ImageShack - Best place for all of your image hosting and image sharing needs
I had to inflate the tire to ridiculously high pressures to avoid that flexing, but then the traction was worsened. Maybe the tire is deliberately designed to do this and that would also explain the presence of the “sharkfin” grooves on the sidewall. I doubt it was because the tire was wider, because I have also used Goodyear F1s size 225 and they didn’t exhibit this behavior. Too bad I didn’t keep these tires long enough to test on the track.
Later I found on a specialized forum that while most tire sidewalls flex uniformly, Michelin tires have the weakest part of the sidewall near the bead, while the rest is relatively stiff.
Of course all this is of little concern to the average driver who only wants to travel from point A to point B. Simply stick to the factory recommended pressure and that’s it. But, yes, different tires have entirely different construction and require different pressures. To get the most out of them you have to find the optimal pressure by trial-and- error.