I haven't checked the speedo accuracy with the new tire with GPS since it was installed. It's not noticeably further off.
I went with the smaller tire assuming that a smaller tire will have less rotational mass, giving me better acceleration and better mileage. I am not worried about traction. I get the impression Brammo went with 180mm for looks, not for performance.
I run upper 30's for pressure in both tires. Motorcycle tire pressures are very subjective. I wouldn't be worried running as low as 30 or as high as 45, but there is a tradeoff between mileage, traction, and tire life.
