Hi Michael: Reliable is a not a great term for most services in rural Mongolia, but that said 72 liters between you will be more than enough for any distances between fill-ups in Mongolia. Just make sure you top up as often as possible (at least in the provincial (Aimag) capitals), because although every little town has a fuel station, they sometimes run dry a few days before the next petrol tanker comes through. As long as you are staying on the major tracks across Mongolia you will rarely go more than 100 km without a chance to buy fuel, and almost never more than 150 km. I have a 43 L tank and never had to use more than about 20 L of it before finding fuel.
In the list of towns you have all of them should have at least a basic fuel station, and the larger ones (Ulaangom, Altai, Moron, etc.) will have fairly reliable fuel supplies.
Scott
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