Jotul 370 has a 61% efficiency rating. So the input btu's will be higher than the output btu's.
86 gal x 91,000 btu's per gallon = 7,826,000 total btu's.
7,826,000 / 24,000 btu stove output =326 approx. (326 units of 24,000 btu's)
326 / 4 hours per day = 81 approx. gal.