“The BMW i3s also sits on 20″ wheels, rather than the standard car’s 19″ wheels. On a normal car, that wouldn’t be a big deal. However, considering how skinny the i3s’ tires are, the extra inch in height actually adds a decent amount of useful contact patch to the road. So there’s more grip now.”

The diameters of the 20″ and 19″ tires are almost identical because the 20″ tires have a lower profile (front: 175/55R20 vs. 155/70R19 and rear: 195/50R20 vs. 175/60R19).

Contact patch area depends only on a tire’s inflation pressure and the weight that it supports. Tire size plays no role. Assuming that each rear tire supports 750 lb and that its inflation pressure is 41 lb/in2, the contact patch area would be 750 lb / 41 lb/in2 = 18.3 in2 for any tire including both the 20″ and 19″ tire.

So the 20″ tire has no greater contact patch area than the 19″ tire. However, the i3s has 20 mm wider tires compared with the i3, so the contact patch shape of the i3s tires is wider but shorter longitudinally. The wider contact patch plus the fact that the 20″ tire is a summer tire while the 19″ tire is an all-season tire explains the better grip even though the contact patch areas are identical.