1992 DTM was governed by the FIA Group A rules and between 1993-1995 they had more freedom which was derived from the FIA Class 1 touring car rules. The official FIA technical regulations for 1992 Group A outline the fuel tank regulations. Currently in RaceRoom the maximum fuel tank size for these vehicles is 60 litres which is incorrect. Here is the technical regulations below. I have also seen various sources suggesting the BMW had a 110 litre fuel tank and the Audi V8 Quattro had a 120 litre tank fitted..
DTM2.5 cars also raced non championship at 24h Spa and Nurburgring at that time. https://www.racingsportscars.com/etcc/entry/Spa-1992-08-02.html I am not sure if they swapped out the tank for a bigger one or RRE is wrong. I seem to recall some DTM races lasting more than 40 minutes back then too.