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Using a class 101 DMCL (I think) as an example, it weighs 32.5 tonnes and has about 65 seats. So if we assume an average of 80kg per passenger fully laden that would be an additional 5.2 tonnes of weight or about 16%. So a fair chunk extra.
(Not sure if the quoted vehicle weight includes fuel - if not a full tank adds about another 0.7 tonnes)