It could be all sorts of reasons, engine mapping, design of combustion chamber. Many on here are more technical than I am.

It can be something as simple as tyres. When the Jaguar XE came out, there was a diesel that was 99g/km by having (amongst other changes) a certain spec of lower rolling resistance tyre other XE's didn't have

Presumably your Fabia is not fitted with a modern TSi engine. Our 2018 Fabia TSi 110 PS has a CO figure of 99 but since it was registered after April 1 2017 we have to pay the same VED as any petrol car.

Before the Fabia we had a Nissan Note DIG-S. Its a clever engine which uses a supercharger which decouples and can change from Otto cycle to Atkinsopn cycle as required , like you say its 99 CO's and costs £0 a year VED (providing it was registered before the rule change in April 2017).

So both engines are 99 CO's and both average about 48 mpg overall and just over 60 mpg on a long run. But give me the TSI any day. The DIG-S needed 4,000 rpm on the clock to get going and it was pretty noisy doing it, the TSI only need 2,000 rpm and is much more refined.

If you are swapping get a Fabia 1.0 TSi registered before 01 April 2017 but you will never save the money you spend changing if its just to save some VED.

I do wonder if any retro fit could remedy this

The answer is simply NO. The figures are certified by the manufacturer and if it were possible the cost would outweigh any savings.

Edited by skidpan on 10/02/2024 at 13:58

I suppose if you go to VW , ask them if they can retrofit a Nissan/ Renault to your car , I’m not sure what their response will be ? Sorry , but not really sure what you’re asking .

The DIG-S engine is in no way related to any used by Renault. Its a Nissan engine and as far as I am aware was only used in the Micra and Note. Renault 1.2's use turbo's, this is supercharged.