Speedometer cluster question

If this were my car I would find out what speed the car is actually going at some easily recognizable O'clock position. Say 12 O'clock. Perhaps use GPS. Or time the time it takes to go between expressway mile markers.

To find out IF an over lay will work look at the speed displayed at 12 O'clock (80 MPH for the 150 MPH GT) but the car's actual speed is 65 MPH (for the 120 MPH V6).

https://lmr.com/item/LRS-17255W99/99-04-Mustang-46-White-Face-Gauge-Kit

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https://www.allfordmustangs.com/forums/v6-tech/304608-my-odometer-goes-off.html#2657361

The other way to determine if a new overlay will fix the problem is by comparing the amount that the speedometer is off. The error should be a fixed ratio of 150 divided by 120 which is 1.25.