Don't read the manual
ianjaytee said:
Got a trek bike manual says 60psi tyre says 120psi how do I work out the correct tyre pressure?
Bike manuals these days have little to do with the bike you're actually riding. They print one manual for a VERY wide range of bikes. The 120 is the maximum rating for the tire and would likely result in an uncomfortable ride, poor traction, faster tire wear, and maybe increased rolling resistance on rough surfaces.
The general guideline is 90-100 psi, and if you get pinch flats, then you need to modify your riding style or get wider tires.