Car Tyre Pressures-checking?

[Morganic]

Before setting off should car tyre pressures be checked with the van attached or with it unattached?

 

[JTQ]

I am not convinced the 100 odd kgs makes much difference. I do it with the car solo as the handbook does not say otherwise and thus no idea of the hitch load.

What will make a big difference is to make sure you do it with cold tyres as the figure given takes into account the significant pressure rise when normally running hot. Somewhat makes nonsense of air pressure pumps at service stations, unless that service station is a very short distance from where the car was stood for a couple of hours.