"Or ask the Mustang GT I topped out with who passed me by about five miles per hour. What's their top speed? 145 or so...?"
Car& Driver clocks the new GT at 138 MPH.
And those speedos can be WAY off (have seen a write up on this with the Viper being >20MPH off at 120 HPH and >30 MPH off at 170MPH.).
This really is a stat best gleaned from the mags. You must (like the major mags) have calibrated equipment, a long/level road, and 2-way runs to factor out wind. Cars typically go a few MPH faster when broken in so this can contribute to minor variability as can car to car variation. But the test driver can't really alter the results. Modified cars - who knows but above 130MPH it takes a good HP boost (or better yet, a drag reduction - the opposite of what people usually do with big tires and big wings) to add just a few MPH. At 150-160MPH it takes even more and here you see alot of pretty powerful iron top out. Car has to be real slippery & strong above 160.
