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OP - I agree. Every car I've had prior to the E88 recommended somewhere around 30-32psi for normal use. I used to run 42PSI all-round on my subaru, higher pressures give more direct handling - a the expense of accelerated shoulder-wear - and less overall grip when you get "over-inflated".
My E88 recommends 48/51PSI for a fully loaded car. I'm assuming that's because it came with runflats.
Since I'm running vastly different tires than the car was expecting (non-runflat MPS4's 225/40 & 255/35) my tire sizes aren't even listed on the plaque.
I had tried running at 48/51PSI and it's just as you say - pretty harsh. ..
So now I run what the tire shop wrote on the book when they sold me the tires - 36PSI at the front, 34PSI at the rear.
Handling, Grip and ride-comfort all seem better than with the ridiculous pressures recommended by BMW.
I'd say you can't go wrong with the factory specs if you're running factory tires; but once you change from factory tires & rims to something else - play with the pressure and see what feels right.
Also - most cars have different pressures for unloaded (driver only) vs fully-loaded (4 passengers+suitcases) car. If it feels harsh on the fully-loaded pressures, check what your unloaded car pressures are, and run them for a while.
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