15% is the industry standard for "number of 4x4s that actually go off-road".
Here are some sources that tend to support that figure:
From MSN Autos:
"For years industry officials have estimated that fewer than ten percent of sport-ute owners ever go off-road."
"But consumers' off-road-use data can be skewed somewhat, depending on the definition of "off-road." Auto industry researcher AutoPacific Inc. of Santa Ana, Calif., found 13.4% of sport-utility drivers said they "go off-roading for recreation" and another 21.7% use their vehicle "to get to off-road camping, fishing and hiking sites."
"Where 15% of men SUV drivers said they go off-roading for recreation, 10.8% of women said they do, too. And while 24.1% of men said they use their SUV to get to off-road camping, hiking and fishing sites, 17.9% of women do the same."
- "In a world where only 15% of SUV owners ever go off road, Jeep says 60% of Wrangler owners do; for Rubicon models, its 80%. Those percentages are the industry's highest, says Kevin Metz, Wrangler brand manager. (Next highest, he says, is the Hummer H3, at 30%.)" ConsumerGuide Automotive
According to Maxim Online (http://www.maximonline.com/maximwear/articles/article_324.html):
"A 1996 survey conducted by J.D. Power and Associates found that 56% of all sport-utility vehicles never even leave the pavement and only 5% go off-road with any regularity."
- "Not that many people really take their sport-utilities off-road. Roughly four percent to seven percent of Land Rover owners do." About That Car
"70% of the customers are seeking parts that will help achieve a different LOOK for a vehicle first: grill guards, 37-inch tires and lift kits are popular items for those customers."