irc said:

Sticking 165/65 r14s tyres on, and using thicker engine oil would probably cause it to fail the emissions tests they ran when homologating the Eco

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No probably about it; the Euro4 Eco's definitely wouldn't have made the 119g/km CO2 cutoff for the £30 VED band with different wheels/tyres.

The Eleganza with its alloys and wider tyres never made the cutoff and stayed with the higher RFL until it was dropped from the range. Fiat didn't offer alloys as standard on the Eco until the introduction of the Euro5 engine in late 2010, which made it was possible to fit them to the Dynamic and still make the £30 cutoff.

That said, once the car has been homologated, I can't imagine the powers that be being seriously concerned about a small change to the tyres/wheels. The RFL band is determined by the CO2 level produced by the car they submit for homologation testing, and isn't updated by subsequent minor aftermarket modifications by the end user.

If you're seriously concerned at some moral level about keeping strictly within the spirit of the rules (and for sure most major car manufacturers haven't been), then you do need to stick with low rolling resistance 155/80 tyres and the appropriate synthetic oil. And you'd better not turn on the A/C; that will push the car way over 119g/km.

the 155-80-13 tyres have a clear rounded shape that should be quite good for aerodynamics. The 195-45-15 fitted to the 100HP have a clear square shape and grip well, but air flows around them like a Luton van. The 14s will be somewhere between. I very much doubt the average driver will notice any fuel consumption difference between the 14s and the 13s but they will for sure notice the better road handling.