I just checked mine over the weekend with a gps. I was able to hit 58.7 mph. This was on my 2000 Sportsman 500 (non-ho). The wind was less than 5 mph. The street was flat and made of a tar blacktop. The gas tank was fully loaded as well as about 15-20 lbs of other supplies. I weigh approximately 250 lbs. Wind resistance did play a part as when I sat up straight, I lost almost 1 mph on the top end.

It is also worth noting that, while I do have some problems in my steering linkage like a loose rivet, the ride was perfectly smooth at top speed.

 

Mine hit 61 GPS verfied MPh with a 1 mile stretch on a dirt gravel road it felt much faster. theres something wrong with yours could be alignment or air filter or clutching.