Arriving at Malpensa Airport, the 31 mile drive into the city center will take around 50 minutes on the A8. Be aware, however, that this is a toll road so plan to accordingly. Linate Airport is closer and will take you 35 minutes on the A51 to travel the 10 miles into the city center. This road also has tolls but you can avoid them by taking Viale Enrico Forlanini, which takes roughly the same time. San Siro is just 3 miles northwest of the city center and will take around 20 minutes on the Via Monte Rosa.
The main routes around Milan include the A1, which takes you to Rome, Bologna, Florence and Naples. The A4 goes west to Turin and east to Venice and Verona. The A8 is the Highway to the Lakes and connects the city to Lake Como, Lake Maggiore and Lugano. These forementioned motorways are connected to the city by the A50, A51 and A52 ring roads. There is a congestion charge for entering the city center (the former walled area) but there is plenty of parking around the ring road network to avoid having to enter this area. Or you can wait until the congestion charge doesn't apply such as during the weekend or between the hours of 7:30 pm and 7:30 am. Your rental vehicle will take you to some wonderful sites around the city, including the fabulous Duomo, the Castello and the incredible shopping arcade of the Galleria Vittorio Emanuele.