I spoke with a friend of mine who told me that 7 pounds of overinflation (going from 33 to 40) will not cause any adverse effects to tire wear, but will do wonders for your handling. The downside, I'm told, is that they will ride a little rougher.

Since going with non-runflats, I have noticed that the ride is nicer, but it was never bad to begin with. I tried out the tires at 40psi and noticed that my gas mileage went up pretty dramatically as well.

Is there anything wrong with doing this?

 

Your MINI will do fine with tire pressure of 38 front and back so at 40 psi you have about 2 pounds higher than the cold recommended pressures for four people.

Most likely you will be fine at 38 to 40 psi for daily use. Air will leak out so you'll be OK over time.

More pressure usually means your sidewall sides of the tread will be in less contact with the road and you'll tend to wear the center part of the tread more. If you chronically over inflate you'll see rubber is worn there but with the MINI we tend to drive corners faster and wear out the outer edges of the front tires. It's likely you'll be OK at 40 psi.

Ride will be firmer. If you want smoother then 34 psi is better. I don't think that handling will be that good. You get less rollover when turning hard. you'll be using a little less of the full tread of the tire. Usually you'll get the best performance running about 37 psi cold.

I've run tire pressures from 33 to about 39 psi on various MINI wheels and they all work to some degree. Nothing that major if you do only street driving. Most of the time I run 35-37 psi for daily driving.

Try out the 40 psi and see if you can tell any difference.

 

Generally, more pressure than the manufacturer recommends increases the chance of blowout from impact if you run over something at speed, but most performance tires are rated for 44psi max (cold) or above anyway. Besides the reduction in rolling resistance, it also tends to improve turn-in at the expense of reducing grip, traction, and self-centering.

Less pressure than stock tends to produce a smoother ride and increases the chance of bending a rim if you run over something. It also reduces the maximum load capacity and safe maximum speed before overheating the tires. FWIW I get massive wheelhop from runflats at 30psi or below, enough to worry about breaking something, and seem to get best 60' results with around 35.

While many cars do respond well to increased pressures (most notably to produce more even treadwear on cars with poor camber settings and 70+ series tires) and for example people at the Toyota Previa forum report <15,000 mile life from their stock runflats if they are not set at 40psi, a more elegant solution would be to correct the camber in the first place. However there are other reasons auto manufacturers carefully select alignment settings and tire pressures: we learned during the Firestone litigation that 1st gen Ford Explorers are speced for just 26/26psi because as far back as 1988 it was known that the UN46 Explorer prototype would lift two wheels during violent maneuvers at 55mph when the tires were inflated to 35psi. That would likely have earned it a "not recommended" from Consumer Reports--far less acceptable than the fuel economy penalty at 26psi, so 26psi F/R was listed specifically to reduce cornering traction to prevent rollovers. Yet people at the Explorer forums still complain they can't find anyone who will mount 20" tires unless they bring them in a different car...

It depends on personal preference and the tires if these trade-offs are worth it to you, but I expect most people feel the MINI turns-in sharply enough and may even be a little twitchy or prone to tramlining already on the runflats for it to be worthwhile. Some people have reported that more pressure in non-runflats gave them back some of the "on-rails" direct steering feel of the runflats.

 

You get better fuel economy because the sidewalls flex less. However, you may not be using the full width of the thread evenly. That would lead to less grip and you will wear the tread out sooner.

 

some tires begin loosing traction and stability when pumped too high.

 

On my S, I run the recommended 35 or maybe 36 when canyon carving. On an interstate trip with the wife I will back it down to 33-34 for a more comfortable ride with the runflats..Just my 2 pennies.