Conny,If you have a 2-speed rear-end,as I'm pretty sure you do,(6500-Series,probably 18,000 gross),you may have "her" set in the "low-side",(rear-end gears). Mine was set on the "low-side",and my top speed was around 40-45,if I pushed it. I found the answer here on Stovebolt,so I thought I should pass it on. Hope this helps. B.T.W... Now I can run comfortably (without over-revving the engine) at right at 55 M.P.H. Fast enough for me,but I don't use the Interstate,either.