In the last couple of years, a number of states have begun capturing the mileage of the car at various events. With only one mileage figure in the official state records, you don't really have much to say that it is correct. Find out if the owner had the car serviced at a particular dealer, and trace the mileage through if possible. Regardless of the source of the mileage, you want to see a coherent string of mileage figures that gradually increase to the present reading on the odometer. Some sources obviously have more credibility than others. You can bet that the dealer's record shows the indicated odometer mileage at any given time, even if the indication is inaccurate.
The 21,000 figure would not be accurate if the speedometer was replaced some time prior to that reading... Sometimes there is a sticker placed on the car by law that indicates the speedo was replaced. It would be simple to remove it and allow a lower reading to show up in the record without explanation.
As to tires, at the present indicated mileage you should either have very bald ones, or pretty new ones if they were replaced. The factory set should have lasted about 30K without abuse.
It is very difficult to detect mileage from inspecting the car, however, which is why it is so lucrative to roll the mileage back. From what I've read, 1 in 3 or 4 used cars has been tampered with.
I knew a guy that had a 4 year old Nissan pickup that would routinely drive 600 mile weekend roundtrips to see his family. He'd disconnect the speedo cable and when he sold it, bragged that it had 30,000 miles not showing. The Maxima's speedo has been electronic for the last decade or more.
Does anyone on this group know if it is possible to disconnect the speedo without screwing up other mechanisms? (I'm not encouraging anyone to post actual directions.)
JM