I would say the sportster would be faster fo the first few miles. After 300 miles the Road King would probably have about a 150 mile lead... so I guess it depends on how far you are going.
ive had both an 07 rk and an 05 1200xl sporty. the sporty is faster off the line obviously. it might even be faster in the top end too, but 75 mph on the sporty feels like youre strapped to the wing of the space shuttle. the road king on the other hand is only cruising at 75mph. i feel like i could fall asleep on the rk as where there would be pucker marks on the sporty seat when i got off of it. two totally different bikes. both are fun in their own ways.
I would say the sportster would be faster fo the first few miles. After 300 miles the Road King would probably have about a 150 mile lead... so I guess it depends on how far you are going.
I have an 05 1200 custom and a 07 roadking. With riding both back and forth I think the Road king will take it hands up from the start. Even with the stage 1's done on both.
If I wanted speed, I would have bought a vette. lol I will take my rk custom every day over the sportster. But like my buddy who had a sporty said, ' it's (sporty) like sitting on an engine. Purely for speed'...Just not for me. At 350 lbs, it would look like a gorilla on a tricycle.