Also, there are seasonal variations in gasoline blend with more of the volatile, lighter fractions being used in winter and more of the less volatile, heavier fractions being used in summer. The heavier blends tend to produce somewhat greater fuel economy which tends to be offset by more use of air conditioning in summer. Also, in a lot of cities, methyl ether and/or ethanol are added up to 10% of volume as an air quality improver. Both of these additives tend to reduce fuel economy.