Running off-spec tire pressure causes uneven wear. Lower pressure typically causes more wear close to the tire's shoulders. Higher pressure usually causes more wear in the middle of the tread. While it is possible to "tune" a certain amount of over- or under-steer with minor tire pressure variations, if what you're after is a softer ride, you might be better off buying different tires.
Running lower pressures can give a softer ride and better traction under some conditions (drag racers run with a big sloppy contact patch from low pressure). But running too low will be dangerous at high speed, both for maneuvers and because the tire will heat up. 25PSI is on the low side though on average, people around the world commonly run 5PSI low without knowing it.
It is best to run with at least the recommended tire pressure used. If you find that too rough a ride then 15" wheels and non-runflat tires might be more comfortable for you.
Not sure about the legality in your area. Here failure to maintain your vehicle could be cause of fault in an accident, and that includes the tires.
MINI Cooper Cabrio: now the car with go cart handling really feels like an open go cart! "... the only man that can come home at 3 am in the morning without getting into trouble with his spouse is the owner of a British sports car!" -- Phil Bailey
I've just recently played around with the pressures in mine. Using my ropey old foot pump, 2.0 bar gave a bouncy ride, 2.2 bar gave a harsh ride and light steering, but 2.1 bar is perfect (all cold readings taken first thing in the morning). Oh, isn't that the recommended pressure. If you would like to borrow my pump, let me know, only a few hundred miles away.
Also, I'm no expert, but as has already been said; I would imagine that should you be unfortunate enough to have an accident and your tyres are significantly under the recommended pressure, then your insurers may hold this against you in settling any claim.
A few years ago I did a driving course and the instructor insisted on all tyres being at 34 to 35 psi, irrespective of manufacturers recommendations.
The reasoning was that manufacturers recommend on the low side to ensure plushness of ride. Higher pressures give more accurate steering, better tyre wear, better braking and better fuel consumption, and a harsher ride.
I recently increased the pressure in my crapelli P3000s to around 32 to 33 psi and not only does all of the above seem to hold true but they have also stopped their incessant squealing.
I second Boo's comments. Soft and comfortable have no place in the world of the MINI driver.
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