In the frame with the German Baron on the side of the road in his automobile stating his mileage is 400 miles per Litre. That equals 105mi/gallon. That's quite good... more than anything we have today in 2020. I think the translation may have been off here? Is it supposed to be 400 litres per mile? That would seem to make more sense.
If they were getting 105mi/gallon you'd only need to carry 36-37gallons of gasoline to make the entire cross country journey: 6000km = 3728mi, 3728/105 = 35.5gal. Making the trip a complete cakewalk.