Yaris has just received a face lift which included upgrading the engines to the ones from iQ. There are some speculations regarding the fuel economy for the 1.33 petrol engine. Some think it might get better fuel economy than the 1.0, while others are more pessimistic.

Here are the numbers as they come out when you use the Yaris spec sheet for reference:

Model, CO2, fuel economy:
Yaris 1.0: 118g/km, 5,0 l/100 km = 20km/l (official)
Yaris 1.33: 120g/km, 5,1 l/100 km = 19,6km/l (official)
iQ 1.0: 99g/km, 4,3 l/100 km = 23,3km/l (official)
iQ 1.33: 101g/km, 22,8km/l (deduced)

To deduce the numbers like this does not make perfectly accurate numbers, as the car maker can choose to tune the engines differently for the individual models (just take a look at the HP rating for the engines in the Auris).

When you look at these numbers, it seems odd if Toyota chooses to make the 1.33 emit 113g/km (as most of the articles mention), when the Yaris manages to keep the emissions for the engines nearly identical.

It seems to be possible to keep the 1.33 below 100g/km, and thus free from road tax: just make it emit 2g CO2 less!