You know how the numbers game works. If your car puts out 120g/km of CO2, your road tax costs £120 per year. One measly gram less and you'll save £85. If you're driving, say, a sensible diesel Korean hatchback and you're gentle with your right hoof, that's about 1,000 miles of free driving.
Hyundai is playing cynical. By doing nothing more than lengthening the gearing on the base-spec 1.6-litre diesel i30, the Koreans have managed to improve consumption by a couple of mpgs and lower the CO2 output from 125g/km to 119g/km. Or £85. Other manufacturers, VW and Seat, most notably, have played the squeeze-the-family-hatch-under-120g/km game too, but not quite so cynically. There are no clever aero revisions, no flush wheel trims here. No eco-badging, either.
Bat-like sensibilities would be required to spot the longer gearing. The greener i30 drives exactly like the base-spec diesel - indeed, performance figures are unchanged. You know the story: it's a bit less fluent on the road than the Golf, but it's a bit cheaper - the same as the standard i30 diesel, in fact.
If you really have the urge to Do Your Bit, take a look at lifetime environmental impact, at NOX emissions, at recyclability. If you want to save a few quid on your sensible diesel Korean hatchback - and give the taxman a cheeky stub in the shin at the same time - this'll do just fine.