You can always install different gearbox or change the gears. But if you only change the fifth there will be huge gap in engine revs between gears. Specially in standard gearboxes if you put very tall fiftfh.
Lupo 3L gears are:
1st 3,455
2nd 1,955
3rd 1,182
4th 0,813
5th 0,642 and If I remember correctly that is freewheeling
Final drive 3,333
Crowheel is also driller full of holes to lighten it up and few other modifications to the gearbox.
With my winter tires 175/70R14 100km/h means 1890 rpm. For next summer I will definetely try in the front 155/80R15 which would give 1804 RPM which is right on the sweet spot
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2 liters is doable for sure its just up to the means and list of the modifications needed to achieve that target.