Hi Consipated, the way i see it is , in the extra urban, it's not just lowering the rev, it's also to do with wind resistance, as that works like carrying weight. on a previouse car i owned, that i covered 200,000k in , & it had a trip PC that showed useage as it happens, so i had a good idea what it's fuel useage was, well after i had traveled about 180,000 i replaced shocks and springs i got lower springs. not sure how much lower, but it was noticible. when i went on a trip i was surprised at the economy improvement. it's just the profile of the car when lowered pushes less wind. the new VW golf bluemotion is lowered 15mm & has the engine stop start so never idles also has the alternator cut out to help cut loads on motor. they also do not have a spare tyre, it gets combined 3.8L/100k but the motor puts out less power & torque than the U2 hyundai. i test drove the Ford Festa Econonetic that gets 3.7L/100k & does not have a spare Tyres it's was a borring car with no performance at all.
