According to the FSM 24.3 gal for LWB, 19.8 for the shorty 3.0L, and 15.9 for the 4cyl.

I think mine's about 22 gallons and shrinking. <img src="/forums/images/graemlins/shiner.gif" alt="" />

1990 Monty LS 3.0 A/T LWB. KYB front shocks, OME rears and coils. 33x10.5 BFG Muds. 5.29, 2.85, RD110, RD46. 2 bouncys. Aisins. Monte Disco swaybar disconnects. GenII t-arms, idler arm, UCA's, V45W front brakes. 2in BL.