"This [study] helps us understand what is most important on a Web site," Brown said. "If [an automaker] only has so much to invest, what is the area that you should invest [in] because it has the biggest bang for your dollar on driving brand and purchase intention? So it turns out that visual appeal, ease of finding a vehicle and site organization are the three top areas that had the greatest impact on people actually intending to purchase."

Keynote also looked at the technical aspects of the luxury automakers' sites, including reliability and responsiveness.

Keynote found that the Jaguar, Lexus and Saab Web sites had the industry's best reliability, meaning those sites were highly available and showed little or no downtime. The Lincoln, Acura and Jaguar sites were the best in terms of site responsiveness, which involved testing how fast the sites downloaded and the array of images typical on luxury auto sites.

Keynote said the luxury auto industry has some of the most complex Web pages of any industry it studies, with almost every site using rich media content and Flash. Despite that complexity, most of the luxury auto sites that were studied provided excellent performance for broadband users, Brown said. However, it often took dial-up users close to a minute and a half to download the automakers' home pages, Brown said.

In terms of site reliability, Keynote found a large gap between the leading sites and those that were lower-ranked. The top-ranked sites were available better than 99 percent of the time, while the bottom five sites averaged just 92 percent -- with several of the poorer sites registering more than 50 hours of outage in a single month.