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This may be obvious to you already, but the source of the additional fuel consumption comes from a few sources (in my interpretation anyway) and calling it "on-road" and "off-road" actually confuses the issue. For the game's purposes "on-road" is only possible in a hex that has a marked road. You can, of course, choose to drive "off-road" in such a hex. In this case you are staying off the fastest and most efficient routes -- and that's where the extra fuel burn comes in. Does this mean there aren't roads everywhere else? No, of course not. I think you would have a pretty hard time finding any 10km area of Poland that doesn't have roads. But these are going to be secondary roads, or worse. Dirt/mud in many cases, rather than pavement. Some of them probably heavily rutted, completely washed out, blocked by abandoned vehicles or roadblocks (manned or abandoned, trees dropped across them, etc). Moving on them is going to be inefficient at best and a real obstacle at worse.

The game by default seems to assume that main highways are not this way. In some places (where they were recently cleared for an advance by a division, say) that might be true. In other places I'd think those highways could actually be much worse than secondary roads -- prime ambush points, mined, struck by any number of munitions, you name it.

And then there's true off-roading (for instance, through the woods or just across the fields and so on). I actually think this would be largely impossible for many vehicles in Poland's forests, but if feasible at all it's obvious why it takes so much more time and fuel. Bogging is a likely risk, moving slowly due to security and the possibility of UXO/mines is a real issue, etc.

You are correct. It is actually worse than that as this is for "Open" terrain. If you drive through "Woods" terrain you double it again (it counts as 2 hexes of movement per woods hex). So, a Humvee with fuel consumption 3L per hex would consume 6L per hex if converted to alcohol, 12L per hex if also "Off-Road" and 24L per hex if the "Off-Road" movement was through Wooded terrain. That's a quarter of the fuel tank gone, for just one 10km wooded hex.

I actually had a bit of an argument with my group about this. I had initially interpreted it like you, but they read if differently:

The Rule says:
"When driving off-road, difficult terrain can reduce your speed. A terrain speed factor of x½ in a hex means that you need to spend two hexes of movement to drive into it."

My players said this means that it takes you more time but not more fuel to drive through a wooden hex.
The definition of Fuel Consumption is: "The fuel consumption, in liters per 10km hex driven."
So, if your vehicle's speed is 8 hexes per shift, you can drive 4 hexes in woods. The fuel consumption is based on the 4 hexes (which is the number of hexes driven), doubled for going off-road, so 8 hexes.

I am leaning towards my initial thought about this and that it is hexes driven, not how many hexes of movement that it cost to get there.

edit: And the example on page 147 supports that.

For the day shift, Diaz decides to head off-road, into the woods. There is a road in this hex, so no roll for navigation is needed. She makes a DRIVING roll, but this time she fails, and gets bogged down! The Referee decides that the mishap occurs after having traveled two hexes (using up four of the six hexes off-road travel capacity, due to the terrain factor for woods).
Diaz makes a STAMINA roll (with help from the others) and succeeds on her second try, but due to the mishap, she loses her two hexes of remaining movement, meaning that the M113 cannot move any further this shift.Two hexes traveled in total this shift means another 64 liters of fuel were consumed (16x2 and then doubled for offroad driving).

Well unless the example is wrong, that pretty conclusively shows that the "speed" factor of a hex when off-road makes it slower to traverse but does not consume any more fuel. Conceptually I just have a hard time reconciling those two facts. Driving very slowly or very fast are both less fuel efficient when compared to driving at some mid-level speed like 50 kph (EDIT - actually I think I should have said 50 mph). Picking your way through forest trails would make you run your engine longer and at lower revs and using lower gears, all of which to my mind reduce fuel efficiency. [EDIT - engine efficiency is best in highest gear but at lower than maximum revs for that gear - or something like that!)

Well unless the example is wrong, that pretty conclusively shows that the "speed" factor of a hex when off-road makes it slower to traverse but does not consume any more fuel. Conceptually I just have a hard time reconciling those two facts. Driving very slowly or very fast are both less fuel efficient when compared to driving at some mid-level speed like 50 kph. Picking your way through forest trails would make you run your engine longer and at lower revs and using lower gears, all of which to my mind reduce fuel efficiency.

I agree, even though (based on the example), RAW say something else.

In reality, twice the fuel consumption in terrain vs. roads seems to hold true for most vehicles. MBTs generally have about twice the effective range on internal fuel on roads as opposed to terrain; 4x4 or 6x6 wheeled vehicles generally consume about twice as much fuel off-road as on roads.

In extreme circumstances (depending on factors like tires, exact terrain conditions, driver skill, weather, vehicle, and a hundred others), the factor may creep up as high as 4x or as low as almost parity, but as a mechanical generalisation, 2x is very reasonable (and for tanks and combat vehicles, often quite stable) .

There seems little basis in reality to "stack" terrain factors for mileage (speed is slightly disconnected).
1x on roads, 2x in terrain — any terrain — seems a good rule to apply. KISS.

According to RAW (I think):

Dirt bike: Travel 6/4; Fuel Cons. 0,5
Road: 1 shift, 6 hexes, 3 l
Off-Road x1: 1 shift, 4 hexes, 2 l
Off-Road x1/2: 1 shift, 2 hexes, 1 l

If using the alternative interpretation:
Road: 1 shift, 6 hexes, 3 l
Off-Road x1: 1 shift, 4 hexes, 2 l
Off-Road x1/2: 1 shift, 2 hexes, 2 l

Sure, the assumed RAW rule is probably how you should do this.

Fuel consumption can vary greatly. However it isn't as easy as on road / off road.

Normal cars do better on a highway cycle compared to a city cycle - but it depends on gearing ratios to a degree. Most cars get maybe 25% less economy in stop/start slow speed city driving compared to racing down a highway in high gear. I have a 2002 car and it goes from 500 km range to 400 km comparing highway to the city, and earlier vehicles are affected more as they need more power to accelerate.

Having done a lot of off roading in diesel 4x4s grinding across rough terrain in constant low gear and climbing slopes all burn through fuel. So you'd probably drop another 10 - 25% over city driving.

Carrying a load also burns through fuel, although vehicles designed for load carrying are less effected. Even sitting stationary with the engine running uses fuel.

So there is no easy "crossing this terrain uses x extra fuel" table you can create. As the GM you need to make the call. Given the use of acohol in jury rigged engines that loss of power may well double fuel consumption if you leave a tarmac or graded dirt road and slog across country instead.

Has anyone thought about how to combine on-road and off-road movement in a single shift? Example: Humvee has 10 hexes per shift speed on-road and 8 hexes per shift speed off-road. If I do 5 hexes on-road, then I can do 4 hexes off-road in the same shift (seems logical). What if I do 3 hexes on-road and spend the rest of the shift off-road? Then I would have 7 hexes of road movement left, multiplied by 8 / 10 = 5.6 hexes of off-road movement left. With fractions of hexes I'd probably round down and call it 3 hexes on-road and 5 off-road.