Well Historically your going to need to set up supply dumps, and also most attacking armies seem to peter out after a few hundred km anyway due to out running supply lines.
Current rules seem to indicate that a Combat Veh with a non fusion or the rare fission engine spend 10% of their engines mass of fuel, and get 667 or 1,000km per fuel tank (Fuel cell / ICE), at lest while driving around, though this seems to be cross country and not road travel.
A few examples the 45 ton Goblin has a 14 ton engine, so it uses 1.4 tons to travle 1,000km, at 1,333 liters per ton it gets 1 km per 1.86 liters *
The 50 ton Musketeer Hover Tank has a 10 ton ICE engine, so it has a 1 ton fuel tank (1.33 liters per km)
The 50 ton Vedette has a whopping 25 ton engine or 2.5 tons of fuel I would think not to bad for a unit going at lest 86kph off road or 3.33 liters per km
The Bulldog has a 23 ton engine so 2.3 ton fuel tank to move it at 64kph off roads (~3.06 liters per km)
The 95 ton Ontos has a 33 ton engine moving it at 54kph off road, and I believe has the largest fuel tank/ICE engine of any combat vehicle in the game. Using roughly 4.4 liters of fuel per km
The Condor has a 12 ton engine so 1.2 tons of fuel
The Po has the same engine specs as the Bulldog
* For reference an M1A2 has 1,900 Liters of fuel (roughly 1.5 metric tons worth (3,250lb at 6.5 lb per gallon (of jp8) and 500 gallons in the tank)) and gets roughly 430km per tank on roads, supposedly it gets 0.6 MPG and uses 60 gallons per hour cross country which is listed at having a top speed of 30mph (the listed (and rumored) higher speeds are for road travel not cross country)).
Basically the M1A2 gets roughly 4.4km per liter. At a max off road speed of about 48kph (I do not know what the ungoverned speed is when off roading, though one Army Tank once told me that 30mph is a bit optimistic in his experience, all I can find is that the ungoverned road speeds seems to be around 80mph or 128kph, with a governed speed of 42mph or 67kph). Basically using the governed speeds I would argue the M1A2 is 3/5 (+1 for road travel per the rules, one could likely add the other +1 MP for Tac ops sat Nav satellites).
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As for Combat use rates, theirs no real good value for that, Strat ops dose have the 1/5th of a tank for a scenario with combat, so one could argue that combat consumes 100kms worth of fuel on top of 100kms worth of travel, though one has to ask how long is that combat going to be?
The Battalion Support companies of current US army Armor Brigades (Circa 2007) have a Supply platoon with 12 M978 Fuel tankers, as well as a 22 more at brigade level (~55 total in the Brigade).
Each fuel tanker carrys ~7.5 tones of fuel (2,500 gal)
In any event I would say that it would not be out of the place for the units supply elements to carry enough fuel for a complete reload, so a Ontos Battalion would need 132 tons of fuel for the tanks (3.3 tons x 40), more would be required for the other vehicles in the unit (personnel & cargo carriers, command units, and what nought).
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Per your scenario of a convoy moving 25ish km, I would say as it seems to have saw combat roughly 50ish km worth of fuel, roughly 3 to 6 tons of worth for a battalion of tanks.