Originally Posted by m4a1mustang

Speedometer is wildly inaccurate. It could be off by 10% at high speed in some cases.

It is limited from the factory to 155 mph I believe. If you were truly going faster than 155 then yes, your car has had the limiter removed and might have a tune on it.

Are your current tires shorter than stock tire height? If they are then your speedometer is going to read faster than you are actually going... and at high speeds the margin of error is going to be even greater than when stock. So it's very possible that your 172mph indicated was under 155 actual.