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In this country, there was often summer gas (E0) and winter gas (E10), particularly in urban areas. This was to give everyone a lean mixture in the winter time to reduce carbon monoxide emissions. Mind you, this only really worked on carbureted vehicles. Fuel injected vehicles (at least the 99% of those with a Lambda sensor) will just throw more fuel in to get the mixture right.

Carbureted vehicles shouldn't experience much change in consumption but modern FI vehicles will.

In the UK the petrol blend changes 4 times per year broadly with each season. The cold weather blends have more volatile components to aid starting but lower caloric value per litre. The difference in calorific value is small and similar to the 1.6% difference between E5 and E10.

The biggest impact on winter MPG is not the calorific value of the fuel blend but the increased density of the air which produces a proportional increase in aerodynamic drag. This drag increase typically accounts for approx. double the MPG reduction compared to the fuel blend.