a question about the guage cluster

i went and looked a 88 gt that is for sale today and the speedo was a ford motorsport one that went up to 160 mph. the mileage on that said like 90000. i asked the guy about it and he obviously didnt know what he was talking about because he said that all 88's came stock with a 160 mph speedo. now my question is, is that mileage going to be accurate, i really dont understang the cluster swap so i dont know if that is the cars actual mileage, or just the mileage since he put it in, or the mileage off of another car. if someone could shed some light on this it would be greatly appreciated. thanks
 
like said already it isn't the factory speedo. factory for that year is a 85mph speedo. i think in 90 or 91 is when they went with a 140mph speedo. get the VIN and run a carfax, might help out on the miles it has.