Are old habits dying hard?

U.S. fleetwide fuel-economy figures from last month suggest that some buyers are going back to relative gas guzzlers. New-car miles-per-gallon averages for May fell for the second straight month, according to TrueCar.

New U.S. cars got a combined 23.2 miles per gallon in May, which was up 5.9 percent from the 21.9 mpg recorded a year earlier but down slightly from 23.3 miles per gallon in April, TrueCar reported. The publication indicated that falling gas prices may be causing some new-vehicle buyers to be a little less mpg-sensitive. Average U.S. gas prices reached about $3.90 a gallon in early April before falling to about $3.55 a gallon earlier this month, according to AAA.

When compared to 2011 numbers, though, mpg figures from specific automakers are still showing gains. Hyundai's fleetwide fuel efficiency rose two miles per gallon from a year earlier, while Honda and Toyota each showed year-over-year gains of 1.6 mpg.

New car fuel economy numbers reached record highs earlier this year. March's new cars hit a 24.1 miles per gallon average, up from 23.9 miles per gallon in February and from 22.5 for the 2011 model year, according to numbers (calculated using a different methodology than what TrueCar uses) from the University of Michigan Transportation Research Institute (UMTRI).