Carmakers may continue to make their cars more fuel efficient year-on-year, but with those models selling in proportionally small numbers, actual gas mileage is climbing at a much slower rate.

In fact, gas mileage in new cars actually fell in May, albeit by only 0.1 percent from April, to an average of 23.2 mpg.

According to TrueCar.com, the slight decrease is due to a fall in average gas prices. This reinforces the direct correlation between gas prices and fuel efficiency, which has seen hybrid sales rising as gas prices head towards $5 per gallon.

Though average mpg is currently falling by small amounts, overall fuel efficiency is still above that of this time last year, at 21.9 mpg. In May 2008, the average was only 21.2 mpg.

Perhaps unsurprisingly, high truck sales mean the Big Three currently occupy the bottom rung for average mpg, with Chrysler on 19.8, Ford on 22.3, and GM on 21.5. Highest overall is Hyundai at 27.7 mpg, but Toyota has the highest mileage for cars at 30.3 mpg--up 3 mpg from last year.

“We continue to see increases in fuel economy among the major manufacturers compared to the previous years but as we’ve started to see a decline in gas prices in recent weeks, consumer preferences showed a slight shift toward larger, less-fuel-efficient vehicles,” explained Jesse Toprak, Vice President of Market Intelligence at TrueCar.com.

Cars are getting better gas mileage all the time, but ultimately it's gas prices that will dictate the success of the most efficient models.

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“We continue to see increases in fuel economy among the major manufacturers compared to the previous years but as we’ve started to see a decline in gas prices in recent weeks, consumer preferences showed a slight shift toward larger, less-fuel-efficient vehicles,”