Trye size is not that important, what is important is how the tyre is constructed and hence its load carrying capacity or it's load index number. This is the number and letter after the tyre size, ie, 94 T, 94 equates to the load it will carry and the 'T' at what speed.
You now need to match the actual tyres fitted on your van to the weight of your van to find the correct pressure they require.
you should find the maximum weight of the van on a plate on the 'a' frame or often near the door, you can then match the weight of the tyres fitted from a chart you can download from this link, http://www.arcsystems.biz/manuals/tp.pdf