speedometers aren't underrated, and modding the engine and drivetrain wont affect the speedo reading, unless you swap in a transmission with different gear ratios. i'm not sure what its called on a bike, but the rear axle gear, swapping it would affect the top speed, acceleration and speedo reading. my car's speedo is still set up for either 3.23 or 3.42 gears and i have 3.73's, so at 70 on the interstate it says about 95.