A TL5 Steam Engine (p. HT14) requires 250 lbs./hr. of wood and 400 lbs./hr. of water (and some amount of oxidizer without access to an atmosphere) for an 8,000-lb. engine. That implies that a Fuel Tank system would run a Steam Engine system for no more than 12 hours (depending on how much mass of oxidizer is required).
A TL6 Internal Combustion Engine (p. HT14) requires 8/3 lbs./hr. (and again, some amount of oxidizer) for a 125-lb. engine, so a Fuel Tank system would run an ICE system for no more than 48 hours.
How much oxidizer do we need? I'm going to guess a 3:1 ratio of oxidizer to combustible by mass*. That gives us
3 hours per Fuel Tank of wood† for a TL5 Steam Engine system, and 12 hours per Fuel Tank of gasoline for a TL6 ICE system. As a reality check, the TL9 MHD Turbine (p. SS20) provides two power points for 24 hours on a Fuel Tank (comparable to one power point for 48 hours), so we're not busting any TL barriers here. If I've mis-estimated the amount of oxidizer required, use a different divisor.
I don't know if the idea (or at least the production) of an oxidizer is appropriate pre-TL7 though. That might be why these aren't listed.
* That's
about what the space shuttle uses. For wood and gasoline, well ... it's been a long time since high-school Chemistry, but I can guess the end products of combustion are going to be CO2 +H2O, which is about 3:1 Oxygen to Carbon+Hydrogen by atomic mass. Is two Hydrogens per Carbon about right for wood and gas? I dunno.
† Charcoal might double that endurance (p. LT27), and coal might multiply it by four (p. HT14).