(Reuters) - New cars and trucks sold in the United States are getting an average of 24 miles per gallon of gasoline, the highest ever, researchers at the University of Michigan said on Tuesday.

The average fuel economy rating as shown by window stickers on new vehicles bought in March - including pickup trucks, SUVs, minivans and passenger cars - was 24.1 mpg, the researchers found. That was up 20 percent from the average of 20 mpg in October 2007, they said.

The university's Transportation Research Institute began monthly updates on fuel economy four-and-a-half years ago.

The researchers also said that their index measuring polluting greenhouse gas emissions per new vehicle has fallen by 17 percent since October 2007.

For more on fuel economy calculations by researchers Michael Sivak and Brandon Schoettle, click: www.umich.edu/~umtriswt/EDI_sales-weighted-mpg.html for fuel economy

(Reporting By Bernie Woodall in Detroit; editing by Matthew Lewis)