4th gear is much stronger than 5th due to 4th being a straight 1:1 ratio, 5th has extra cogs as it is overdrive.
I very rarely use 5th when towing any considerable load (manual 300tdi) simply to avoid undue stress on the gearbox, and I have found I can actually get better economy as the engine doesn't have to labour as much to hold speeds, particularly up hills.
Can't help with what the economy should be, consider weight and drag through, and basing a consumption on 1 tank is generally not very accurate.
Seano


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