speedometer question

I have an 80 kz750 twin, and the speedo registers 85. i want to install one that registers more than that and i am wondering if i have to worry about any sort of calibration or differences in how they register speed. is this a legitimate concern or will pretty much any speedometer read accurately as long as it is compatible with my speedo cable? not that i plan on running this fast on any regular basis, but just once or twice i would really like to see where i am topping out.