Seems rather slow to be proper, espicially two NSXs getting that speed or lack there of. The air density would have a small effect but you should have made 170 at least.
Their measuring equipment must be bollicksed up. Next time try and have some kind of porable GPS device that just plugs in to the fag lighter and that would be a reliable way to check their equipment and your indicator.
I have an older portable GPS, (Garmin 100) I take from aircraft to aircraft and put it on the instrument panel just ahead of me and found my installed indicator was off 4 mph. Maybe not fancy but accurate.
From being with Honda motorbikes for many years i've found their mph indicators are usually off a few mph and get worse the faster you go. Not enough to matter but off a bit until the digital ones came about.

Cheers
nigel