13.8 seconds

I subscribe to the two major US motorcycle magazines, Motorcyclist and Cycle World. They are celebrating 100 and 50 years of existence respectively this year. Usually I'm pretty disappointed. Nary a peep about Royal Enfields, just comparos of superbikes followed by flurries of letters to the editor complaining that there is too little/too much coverage of Harley Davidson or Ducati. An exception has been the Motorcyclist section "Me and my bike" which featured a 2002 Bullet owned by a gentleman named  Jim Moy, and more recently featured The Badger that David Blasco has covered in his blog.

But in the March 2012 issue Cycle World has to surrender to the might of the Interceptor. In their article 50 years of  "Do you have any idea how fast you were going?" they note that the Royal Enfield Interceptor in 1965 was the first bike to break 14 seconds in the quarter mile, a feat that will remain unchallenged by a true production motorcycle till the Kawasaki 500 Mach III in 1969. And its top speed of 120 mph will not be broken until the devilishly complicated Benelli Sei entered the scene in 1974. The article even includes a picture of the Interceptor, albeit in 1969 trim (which is cool since it looked nicer than the 1965 model in my opinion).

If Enfield had taken seriously the production of the Interceptor, instead of putting out just a trickle of them, history would have been very different.

I could not find the article online so I include a scan of a nice table with time to quarter mile in green, top speed in red and speed at quarter mile in blue. Click to enlarge.