Driving distance is calculated by the PGA Tour as follows...
"The average number of yards per measured drive. These drives are measured on two holes per round. Care is taken to select two holes which face in opposite directions to counteract the effect of wind. Drives are measured to the point at which they come to rest regardless of whether they are in the fairway or not." The holes that are selected for measurement are ones that most golfers will elect to use a driver.
the current Top Ten...
1 Bubba Watson 313.8
2 Robert Garrigus 306.3
3 Gary Woodland 306.2
T4 5 Angel Cabrera 303.0
T4 8 Nick Watney 303.0
6 Scott Piercy 302.3
7 Dustin Johnson 302.2
8 Charley Hoffman 299.7
9 Bill Haas 299.2
T10 Jonathan Byrd 298.5
T10 John Merrick 298.5
I have been a PGA Professional for 33 years, any golfer who claims he averages over 350 yards is probably exaggerating. However, it is very likely that a golfer with a swing speed in excess of 120 mph can, under the right conditions, occasionally drive the ball more than 350 yards.
Hope this helps.