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As a followup, I researched the tires on the BMW i3 - 155/70R19 84Q / 175/60R19 86Q for the base model, inflated to 33 psi / 41 psi.
5 things jump out at me.
1) These are conventionally sized tires - just unusual in their configuration (large wheel diameter, but tall and narrow.) A more usual sizing would be 175/70R13 with a load index of 82 and a 185/70R13 with a load index of 86.
2) They are Q speed rated. Q is a common speed rating for winter tires, but summer and all season tires have at least an S speed rating. I wonder if the tread compound can't stand the higher speed (heat dissipation), but this car doesn't generate enough speed to warrant an S rated tire. I wonder if the top speed is restricted in some way.
3) They are large in diameter. A 155/70R19 = 27.5" vs 22.7" for a 175/70R13. The important take away would be that the vehicle would have to be designed to accommodate this larger diameter.
4) When I put those 2 sizes into the RR calculator, I get a 24% improvement in RR!!
5) The 41 psi for the rear tires is highly unusual for a SL tire. That ought to result in a HUGE RR improvement. (For those who wonder, the maximum load carrying capacity of an SL is rated at 36 psi (35 for English units), so specifying more than that not only does NOT increase the load carrying capacity of the tire, but should make the tire prone to ride in the center of the tread. I wonder if they did that for handling reasons.)
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