155/60-R15 tires are 0.24 inches (6 mm) smaller in diameter than 160/60-R15 tires and the speedometer difference is 1.1%
www.tacomaworld.com
Here is a size calculator, but the car tire manufactures site should give better dimensions.
I ran darkside on a VTX1800R, the tire would out perform the bike, when I had to replace the rear I went with motorcycle tire, no more having to force the bike over, was so much easier and "natural" to ride, but that's me. these scooter tires are SO EASY to get off for a swap out, and they handle SO WELL, just doesn't make sense to convert to a car tire. The only 2 write up/ reviews I've seen on darksiding a scooter was when the poster was converting back to get the ease of handling back (both Bergmans). But , best of luck with whichever you want to do.