c4 with 2.80's

[John Z]

Back to your original question about going over 160 in a stock mustang - no way. The high air resistance of these cars would require more power than the engine could produce. In other words, you can not wind the motor to a high enough rpm to reach this speed.
If you want to estimate the error in your speedometer, measure the time it takes to go a mile while your speedometer is indicating 60 mph. If it takes less than 60 seconds, your speedometer is reading high. There are mile markers, little green number signs, on the side of interstates that you can use as a measured mile.