Honda TRX420TM Owner's Manual page 162

Tyres
Always check air pressure when your tyres are ''cold.'' If you check air
pressure when your tyres are ''warm'' even if your ATV has only
been ridden for a few miles the readings will be higher. If you let air
out of warm tyres to match the recommended cold tyre pressures, the
tyres will be underinflated. Be sure to check tyre pressure at the riding
site, since changes in altitude can affect air pressure.
The recommended ''cold'' tyre pressures are:
tyre pressure,
front & rear (cold)
Servicing Your Honda
152
front 25.0 kPa (3.6 psi)
rear 25.0 kPa (3.6 psi)