If you buy from a dealer, or from an internet discounter, you order it, specifying the mileage you want it to read. It will be set at the supplier. I don't know if dealers have the equipment required to do the setting.
What I did was order it from Chicago Harley (Zanotti's is good too), asking how long it would take to get it. Knowing my riding habits, I then guestimated what mileage I would have at that time, added 500 more miles, and requested that mileage to be set into the unit. When it arrived, it was a bit early, mileage-wise, so I waited until the mileage was right, and swapped it out...just right. I saved the old speedometer, to give to a potential new owner of my scoot...like that is going to happen...just to show that no funny business was done.
When you order it on line, put the mileage you want in the comments box at checkout, then ask them to email you back to show that they saw it. Actually, I contacted them first, to see what they would need to see my specified mileage. Turns out they would have contacted me if I did not specify one.
On a side note, make sure you fully seat the new tach pin in the connector to the speedo/tach. If done right, you will detect a discernable click. If you don't it will likely come out, or loose, and make you think the unit is bad, down the road. Be careful, the pin does not go in easy, and you don't want to break the wire or pull it out of the pin.
Enjoy,
Rich P