Tire CIRCUMFERENCE determines the precise final drive ratio...so technically, that is whats effecting everything with this...but for simplistic purposes, generally an increase in diameter automatically increases circumference of a circle...
so yeah, what everyone else said accept the first response...A larger tire circumference decreases the final drive ratio (makes every gear uniformly 'taller'), causing the same speed to occur at a lower drivetrain rpm...but the speed sensor(s) doesn't 'see' this change, as they are recording rotational speed at other locations...so you loose speedo accuracy, but only marginally unless you're way out of spec tire size wise...if you're ever worried about it, just use a gps system in areas you're not familiar with where cops typically hang out...the GPS speed rating will remain accurate regardless of all this gearing bs...
despite the speedo issue... I've always recommended going slightly higher in tire size than lower...these cars are already ridiculous rev wise in 5th at highway speed...anything to help lower that is welcome imo...
