That tire is about 30.6" diameter. If your speedo & tach numbers are accurate, you have some incredibly low gears. Is it possible the trans could be slipping?
http://www.richmondgear.com/101032.html
Known RPM:
3500
@ Known MPH:
55
Tire Diameter (Height) in Inches:
30.6
Estimated Rear End Ratio:
5.80