the legalities are none except if you decide to sell it or if your state has a cutoff mileage for vehicle testing.
If you sell it you MUST report an odometer inaccuracy or you can be in a world of hurt. It will also significantly reduce the resale value of your car.
This has already happened however.
Regarding emissions testing, basically it means you might have to get it inspected beyond the mileage you ordinarilly would. Whether you care or not is up to you
Here's the trick though. Buy a speedo cable and install it. Your problems will go away. You're describing CLASSIC symptoms of a busted cable - it'll still make contact from time to time which will make your speedo work, but overall it's busted. You can check this if you want, just unhook it from the transmission (easiest end to get to) and tug on the wire inside - if it's busted it'll come right out and you'll see the frayed end.