[quote="PhilH] I'd always assumed (with no evidence whatever) that the tyre diameter and width reduced rolling resistance.[/quote]
Rolling resistance is more a tire carcass construction, tread design, and rubber compound relationship verses the width or diameter. Low rolling resistance can be added to any tire width/diameter...but, cd is totally a function of width in the airstream. Combining low rolling resistance and a low cd were, I think, the reason they were used on the i3. Tire pressure helps, too, by preventing deflection. It takes energy to deflect the tire which generates heat which is wasted. Some of that heat comes from tread squirm, and the compound and shape of the tread blocks affect that. That energy comes from somewhere...it's not free. Bridgestone talks about it on their website. Nokian does as well on theirs and claims a 30% improvement over 'conventional' tires. There are some tradeoffs, as with most things. The narrow tires/wheels on the i3 decrease some of the barn door effect - you don't have to push as much through the air but allow enough tire patch to provide adequate traction. But, the bottom line is, it has a decent tire patch and thus can hold its own.