How to re calibrate speedo on a 2004
 

Have a 2004 Mach 1 and switched to 18 inch wheels....still running 3.55 gears but now speedo 5 mph too fast.....checked with a gps app

so how to re calibrate?

My friend used a diablosport hand held tuner. That's was years ago though. However, it worked.

Yup, the PCM needs to be flashed and recalibrate to new tire height. A hand held tuner would do the trick, a tuner shop would be able to do it, a dealer should be able to do it for you too.