JEFF B - The 982 has a 4 gal tank, I fill it almost full when I start mowing, maybe 3-1/2 gal, and if I mow the whole yard I'll have to add maybe another gallon to finish. There may be a half gallon left but the gauge reads empty, never really measured, but I have run the 982 for a half hour towing the cart & other light jobs on the "empty tank". It may actually only burn four gal, maybe a bit less.
I had a home-made 8+ quart tank on my old 129 and mowing with it I could mow everything on 3 to 3-1/2 gal. When I had the 5-quart tank on the K241 in the 72 I'd have to fill up about three times to mow the whole yard, so I figured 3-1/2 to 3-3/4 gal. SON made a BIG fuel cell for the 72 when We put the K321 in the 72, held 2-7/8ths gallons. I ran that for about a year and one of the mounts cracked/broke which would have required welding on the tank itself. I'd have to add some gas, maybe half to 3/4 gallon to finish mowing with that tank. I replaced it with a new 8 quart plastic replacement tank. Mowing with that tank, it always seemed I'd run out of the second tank full with about 5-10 minutes of mowing left. I try not to ever run out of gas, the engines run lean, and then die under load, that hot soak is kinda hard on the exh valves & rings.
When SON & I used to both mow, me on the 10 HP 72, SON on the 129 he'd have enough gas to finish, but I'd put a splash of gas in to finish trimming around all the bushs. I could always get about 2 to 2-1/2 hrs of running on that 5 quart tank of gas and we'd get done in 1-1/2 hrs, maybe a bit more but I had some steep slopes to mow along the ditch by the road and needed more gas to prevent running out.
So all the various tractors have used close to the same amount of gas, the 129 w/44A deck being most fuel efficient, 72 & 38" deck a close second, the 982 w/50C a more distant third. But the 982 is quite a bit heavier, has hydraulics, elec. PTO, but it rides so nice & smooth, plenty of room to stretch out, it was the only thing I mowed with last summer.
The long term plan is to rebuild the K241 that came out of the 72 and install it in the 70, with it's slower second gear it "SHOULD" be the fuel economy king of the fleet with the 38" deck off the 72. The 72 has the fast 2nd, which IMHO is just a bit too fast to mow, I've mowed a LOT with it in second gear but it just doesn't do as nice a job as I want.