Hello,
The load rating is determined by the weight, 91 is about right for the stock variant tyres. My 97W was not a science but my preferred requirement. My AXIS rims have a maximum rating of 730kg.
Most rims state the maximum weight on them say 630kg. Of course I am over kill to be safe. Say dry weight of your Stag is 1800kg, then add 5 x 80kg guys and luggage in the back then total weight is easily up to 2200kg / 4 wheels is 550kg per tyre, so your 91w will do fine allowing 615kg. . but better rating is desired in my own view. Plus the speed rating of W is more than enough. .
Load Index - 81,82,85,86,87,90,92,95,96
Max Load/tyre (kg)462,475,515,530,545,600,630,690,710
http://www.toyo.com.au/TechInfoPDFs/Load%2...d%20ratings.pdf
Hope that assists.
Jules