TFL did a study related to this which was cited when Wales set the default speed to 20.
They found that there was minimal difference in emissions between the two speeds. Any benefit of lower drag etc is countered by the fact that the engine is not geared well for 20 so generally it will be the same or only slightly better doing 20. (I am making the assumption here that lower emissions will equal less fuel usage.)
However, the greatest benefit is that there are less cars on the road because it encourages active travel and discourages driving. So comparing driving to not at all is a huge fuel difference.
TFL did a study related to this which was cited when Wales set the default speed to 20.
They found that there was minimal difference in emissions between the two speeds. Any benefit of lower drag etc is countered by the fact that the engine is not geared well for 20 so generally it will be the same or only slightly better doing 20. (I am making the assumption here that lower emissions will equal less fuel usage.)
However, the greatest benefit is that there are less cars on the road because it encourages active travel and discourages driving. So comparing driving to not at all is a huge fuel difference.
I can’t find the exact study I looked at before, but I think this might be similar: https://content.tfl.gov.uk/the-impact-20mph-limits-and-zones-in-london.pdf