For the second year in a row, rental car customers are reporting fewer complaints, according to the results of the annual J.D. Power and Associates North America Rental Car Satisfaction Study.

J.D. Power said the industry’s overall customer satisfaction score on its 1,000-point scale in 2011 was 758, up from 750 a year ago, and that satisfaction increases were reported in all six areas the company measures – costs and fees, the pick-up process, the return process, the car itself, shuttle bus/van service, and the reservations process.

However, J.D. Powers’ Stuart Greif, vp of its travel practice, said rental firms still have room for improvement, “particularly in leveraging technology.” He noted that customer satisfaction could be helped if rental companies came up with a way to integrate customers’ mobile devices within the vehicles so they could enjoy hands-free phone calls, music or navigation. He also suggested that rental firms should investigate gas tanks with digital readouts for more precise estimates of available fuel; that they should digitally extract gas and mileage data from the car when it is returned; and provide real-time shuttle bus arrival times for customers.

The 2011 study found that the average waiting time to pick up a rental vehicle was 17 minutes – an improvement over last year’s 20.5 minutes. However, it noted that in the pick-up process, “satisfaction drops considerably among customers who have to wait more than five minutes to receive their vehicle.”

For the first time, ACE Rent A Car ranked highest in customer satisfaction among all companies; it’s also the first time ACE appears in the rankings at all. Ranking second overall was Enterprise, followed in third place by National.