yea it doesnt mess with the tach. but depending on how much smaller/bigger your tires are from stock your speedo will be off. for example, when i race at the track i run a 205/50/14 tire, thats significantly smaller so my speedo reads like 100 when im only going 90. also tire size changes your final gear ratio
... I'm going to start keeping track of how many times I post this thing, its been at least 10. And he's right, your tach feeds of an electrical signal from your ignition, not anything to do with your wheel speed.