The reason: CO2 and mpg are closely linked - the higher your mpg, the lower your CO2 emissions for any given fuel. Of all the vehicles Emissions Analytics tested, the top 10 most efficient were all gasoline hybrids, led by the Hyundai Ioniq at a whopping 59.1 mpg. The increasingly popular SUV segments sit somewhere in the middle of the efficiency spectrum, with small gasoline SUVs averaging 24.1 mpg, while standard gasoline SUVs only achieve 19.8 mpg. Engine efficiency improvements have helped SUV mileage rise by an average of 0.7 mpg per year over the testing period, compared to market rises of 0.2 mpg. [Extracted from the article]