The WardsAuto Fuel Economy Index fell for the third consecutive month in June to its second-lowest 2012 rating, after registering a record-high 24.1 mpg (9.74 L/100 km) rating in March.

U.S. gasoline prices also tumbled in June, to their second-lowest level since December.

New cars and light trucks sold in June combined for a 23.6 mpg (10 L/100 km) rating, marking a 0.6% drop from May’s 23.7 mpg (9.9 L/100 km), and a 2.1% decline from March’s high-water mark.

The parallel movement of gas prices and average fuel-economy of new vehicles sold, especially over the past six months, appears to point to fickle behavior on the part of U.S. consumers almost presciently adjusting their buying habits to daily changes in the price of fuel.

But while there is a strong correlation between gas prices and fuel economy, some of the index’s change in direction is related to a fluctuating fleet sales mix.

Toyota, the leader in overall fuel economy, and Hyundai Group, which sits near the top of the Index, played larger-than-usual roles in fleet sales in March and April, in a market that already was trending up in average fuel economy.

Both auto makers appeared to pull back fleet participation. Toyota, which earlier in the year seemed to be working to make up for 2011 fleet sales lost to inventory issues, returned to more traditional levels in June.

The auto maker’s subsequent reduction in market share, from 15.3% in May to 13.9% in June, certainly played a role in the month-to-month decline in the market’s overall fuel-economy rating.

Industry-wide, cars lost a slight share in June, making up 50.3% of all LV deliveries compared with 50.6% in May, and earning an index rating of 26.8 mpg (11 L/100 km), down from May’s 27.1 mpg (8.7 L/100 km). Light trucks earned a 20 mpg (11.8 L/100 km) rating.

Small cars accounted for 19.2% of LV sales, compared with 19.9% in May, while scoring a 29.6 mpg (7.9 L/100 km) rating.

Midsize cars, rated 26.5 mpg (8.9 L/ 100 km) for the month, also lost share, accounting for 21.9% of all LV deliveries in June compared with 22.3% in the prior month.

Cross/utility vehicles comprised the largest sales segment, taking 24% of LV deliveries, while earning a 22.1 mpg (10.6 L/100 km) index rating.

Toyota led all auto makers on the WardsAuto FEI with a 27.2 mpg (8.6 L/100 km) rating, well below the industry’s single-company record of 28.5 mpg (8.2 L/100 km) the auto maker set in April.

Overall, Asian auto makers combined for a 26 mpg (9.0 L/100 km) rating, followed by the European brands’ 22.8 mpg (10.3 L/100 km) and the Detroit Three’s 21.2 mpg (11.1 L/100 km).

Domestic-built LVs registered 22.8 mpg (10.3 L/100 km) in June, compared with imports that earned a collective rating of 26.6 mpg (8.8 L/100 km).

The industry rating for the year’s first six months was 23.8 mpg (9.9 L/100 km), up 4.4% from like-2011 and a 13.8% increase over the fourth-quarter 2007 base period for the WardsAuto Fuel-Economy Index.

jsousanis@wardsauto.com