The popularity of new trucks lowered the average fuel economy of new vehicles sold last month.

With trucks accounting for more than 56% of all vehicles sold in the U.S., the fuel economy numbers of new vehicles sold in the U.S. slipped last month, according to researchers at the University of Michigan Transportation Research Institute.

Based on the number collected by the Environmental Protection Agency, the average fuel economy of cars, light trucks, vans and utility vehicles purchased in April was 25.2 mpg, down from 25.4 mpg in March.

“This drop likely reflects the increased proportion of pickup trucks and SUVs in the sales mix,” said Michael Sivak, a research professor at UMTRI. Overall, fuel economy is now down 0.6 mpg from the peak reached in August 2014.

Fuel economy is up 5.1 mpg from October 2007, the first full month of monitoring by Sivak and colleague Brandon Schoettle.

However, the average vehicle fuel economy during the first seven months of this model year (October 2014 through April 2015)—25.3 mpg—has stayed the same as during the preceding model year.

In addition to average fuel economy, Sivak and Schoettle issued a monthly update of their national Eco-Driving Index (EDI), which estimates the average monthly emissions generated by an individual U.S. driver. The EDI takes into account both the fuel used per distance driven and the amount of driving—the latter relying on data that are published with a two-month lag.

During February, the EDI remained at 0.82 (the lower the value, the better) for the second straight month. The index currently shows emissions of greenhouse gases per driver of newly purchased vehicles are now down 18%, overall, since October 2007.

Meanwhile another report from U-M indicated that consumer optimism rose last month to its second-highest level since 2007—only below the January 2015 level, according to the University of Michigan Surveys of Consumers.

(Drivers getting break this year on cost of ownership. For more, Click Here.)

Conducted by the U-M Institute for Social Research since 1946, the surveys monitor consumer attitudes and expectations. The data are available non-exclusively via Bloomberg.

The Sentiment Index has recorded a higher average level during the past five months than any time since May 2004, according to U-M economist Richard Curtin, who directs the surveys. Although recent gains in jobs and incomes have prompted the most favorable personal financial expectations in eight years, consumers’ assessments of their financial situation are still far below the peaks recorded in earlier decades.

Consumer optimism has become increasingly dependent on the persistence of low inflation and low interest rates as well as slowly improving prospects for jobs and incomes. Despite a slower pace in the 1st quarter, personal consumption spending is expected to post a 3% gain in 2015.

(Click Here for details about the steady rise in gas prices.)

“While nearly two-thirds of all consumers anticipate rising interest rates during the year ahead, they nonetheless expect very minimal increases,” Curtin said. “For a successful Fed policy, consumers must judge whether the negative impact of higher interest rates will be easily offset by the positive impact of expanding jobs and incomes.

“That tradeoff accompanied rate hikes in the past, but it has never been undertaken when interest rates have been so low for so long. Along with other changes in consumer evaluations that have recently occurred, consumers are likely to have become more responsive to much smaller changes in interest rates than in the past,” he said.

(To see more about April sales rising due to SUV and truck sales, Click Here.)

Personal financial gains were expected by 37% of all consumers in the April survey, the highest proportion since April 2007. The improvement was due to both higher income expectations as well as more job opportunities, although the anticipated gains in incomes and jobs remained quite modest.