The speedometer reads wrong, bounces around from 20MPH to 80MPH then to 0MPH. Nissan replaced with a used unit, now my mileage is wrong by 7k over!! Damn Nissan!!

The speedometer reads 110mph on the freeway. The needle sticks at 20-25mph when the engine is turned off and at "park". Was told if I replaced it, I would have to pay for the part first. They would read my current miles on the van and send the order to New York. I suppose I can't drive the van after the miles are read? My Nissan repair service would install the new part when they receive part in 2 weeks. I know I'm not the only person who is having this problem. Shame on Nissan.