Somewhere on the car (usually around the driver's door lock clasp) there should be a sticker which gives the recommended tyre pressures for the car, and that will list the tyre sizes that the car was intended to have from the factory.
See if your tyres sizes match any of those - or look in the owner's manual. The manual for your car will probably be here: www.bmwsections.com/docs/3series/
It looks like 225/40 front and 255/35 rear is correct, the speed rating is W.
A previous owner may have fitted aftermarket wheels, in which case you have to choose tyres that fit those wheels.
We have a 420i with similar size runflat tyres, and have found that Pirelli P7 work well. They were the factory-fit and when it came to replacement they were the cheapest 'brand name' runflat too. We got just over 25,000 out of the rears and 30,000 from the fronts.
You can shop prices by size using BlackCircles.com.
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