Continental only makes tires in the OEM size for the front. They don't have 175/60R15 rear tires. I've also verified that the 175/55R15 size they offer will NOT work on the I-MiEV with the OEM front size (these are offered for the Smart Fortwo).
In my experience (as I've had all three brands), Dunlops offer the best combination of ride, handling, range, and noise. The Yokohamas are quiet and handle really well, but cause a drop in range. The Continental front tires are slightly noisier and don't handle as well, but I had better range (+5 miles) and was consistently getting more than 4 miles/kWh.
The front tires on Bear are nearly gone. I think I'm going to swap over the Yokos that are on Koorz and try to quantify the range drop. Dropping $600 each year on rubber is getting old. You'd think with a massive influx of gas traffic, there'd be more than enough funding for some half-decent roads