Why does my odometer speed is 120 on my 1987 Grand National and other GN cars are 85.

Back in the days of the 55 mph speed limit, there was a law...see below Speedometers[edit](55) emphasized on a Festiva dashboard 55MPH is emphasized on this dashboard. On October 1, 1980, in a regulation that also regulated speedometer and odometer accuracy, the National Highway Traffic Safety Administration (NHTSA) required speedometers to have special emphasis on the number 55 and a maximum speed of 85 mph (137 km/h). However, on October 19, 1980, NHTSA proposed eliminating speedometer and odometer rules because they were "unlikely to yield significant safety benefits" and "[a] highlighted '55' on a speedometer scale adds little to the information provided to the driver by a roadside speed limit sign."[37][38][39] Sometime around the time your cars were made, the law changed or was about to be changed and some performance vehicles like the GN and Corvette changed the reading on the speedometer to have a higher upper limit. I suspect you are looking at a speedometer built before the change and after.