Ok so for anyone else who wants to know what to do, this is what i did and works a treat.
Calculate the difference between your current speed and actual speed. For me, my speedo was at 100, gps was at 102, so a 2% difference due to a larger tyre, ie. 275/35r19 instead of 275/30r19.
In vid block scalers, find tyre size, should be 0.78 etc and minus 2% off this value.
Reflash, and all good to go.