fit bigger tires is the easy fix.
harder fix is to recalibrate the speedo. it depends on what year your ute is.
late models utes sometimes do a trick where they are correct at 50km/h but 10km/h out at 100km/h.
It's common with all vehicles manufacturers. Protects them from being sued when you get done for speeding and you defence is the manufacturers speedo was inaccurate.
Use a GPS for speed measurement. It's accurate to about 1% - so at 100km/h indicated by the GPS, you could be travelling at anything between 99 and 101 km/h.
Yeah I use my phone for speedo. On my old trooper you could change a small wheel on the gear box to suit the tires. But I guess buying bigger tires is the easy solution