I've had the car since January 2014 front driver side tyre looses pressure couple times a week but other 3 are fine. Suspensions are fine I've replaced all 4 tyres with Michelin pilot super sport about 8 months ago still having the same problem. What could it be?

 

Was the same wheel loosing the pressure with the old tire and new tire?
May be there's a hairline crack on the wheel. Or that wheel is not properly sealing with the tire.

 

Could also be a nail or screw in the tire. They can be very hard to spot once the head has ground off. If the tire is leaking that much, take it to a tire guy and they will place it in water to look for bubbles. You can do it yourself if you have a plastic baby pool or other large tub.

 

Have the valve stem changed out if it is the same tire having the issue. Also make sure their is no nail or foreign object stuck in it. If that does not work then you need a new tire.

If it is different tires then take it back where you got it from and have them redo the tires. Problem solved

 

I’ve had the tires taken off and edges of wheel cleaned and resealed tyres about a month now and the issue seems to have been solved. Also noticed a couple MPG increase and better handling.
I use to have the pressure set at 2.3 BAR rear and 2.1 BAR fronts but have been increasing pressure by 0.1 BAR per week currently running at 2.7 BAR all round and it seems to me as though it drives and get better MPG on higher pressure. What are others running at on a similar setup?

 

05 530i 245 40x18 tires sports suspension: 36 psi front, 44 psi rear