The pioneers of ultra low-CO2 emission cars have been constantly pushing the boundaries to levels beyond those that could have been predicted just three or four years ago, and Seat has certainly been up with the advanced party.

Along with Ford and parent company Volkswagen, the Spanish brand was first to get below 100g/km for a supermini, with the Ibiza, and while that model is to get a version at 89g/km later this year, its bigger brother Leon, the lower medium sector model, is now in an exclusive club below 100g/km.

Only the VW Golf, Ford Focus, Audi A3 and Toyota hybrids the Auris and Prius can also get under that emissions point at this stage, and there aren’t any other rivals in the pipeline. The likes of Peugeot, Citroen, Renault, Vauxhall and even BMW aren’t on the verge of cracking the two-digit emissions figure.

And it’s going to become more important as a boundary. At the moment the only real benefit is VED exemption, saving £35 per year, but in less than two years only those models under 100g/km will qualify for the lowest 10% (13% for a diesel) benefit-in-kind tax band, and London mayor Boris Johnson is looking to introduce congestion charge exemption for sub-100g/km vehicles, potentially saving £10 per day for those drivers needing to enter the zone. And that’s all without even mentioning the official average economy figure of a huge 74.3mpg.

The good news is that to drive, the Leon is virtually unrecognisable as a low-emission model. The 105hp 1.6-litre engine doesn’t feel like it’s been tuned for economy, offering the kind of diesel shove you expect from a mid-range model rather than an economy benchmark. And specification isn’t compromised, with the SE trim driven here benefiting from such luxuries as alloy wheels, cruise control, dual-zone climate control, electric folding mirrors and rear privacy glass. And it’s that kit list that nearly swung our favour towards the Leon over its VW Golf rival, even though the Golf has a slightly lower pence per mile figure and the more prestigious badge. The Volkswagen does without all of those items for its higher P11D price, but has a residual value of 38.1% against the Leon’s still-reasonable 34.3%, which beats either the petrol-electric hybrid Auris (32.6%) and the rather expensive Ford Focus (25.0%) listed below.

The Leon is a practical, competitive and appealing low-CO2 model that pulls off the compelling trick of appearing compromise-free and offering running costs that maximise potential savings for drivers and businesses.

Seat Leon 1.6 TDE 105 SE Ecomotive
P11D price £18,085
Model price range £13,975-£25,205
Fuel consumption 74.3mpg
CO2 (tax) 99g/km (13%)
BIK 20/40% per month £39/£78
Service interval 10,000mls
Insurance (1-50) group 15
Warranty 3yrs/60,000mls
Boot space 341 litres
Engine size/power 1598cc/105hp
Top speed/0-62mph 118mph/11.5secs
On sale June 2010
Score 8/10
Verdict Cost-effective, interesting
ultra low-CO2 alternative