Scotty, look in the Technical Q&A section, for the topic "RPM/MPH". My speedo is bouncing wildly and I was looking for some tach readings that I could equate to mph, so Rob was kind enough to use his GPS and run some calculations. According to his figures, 60 mph = 4500 rpm. Stock speedos aren't terribly accurate even when working properly, and read high, so I'm betting your 4000 rpm is actually closer to 55 than 60.
That is, unless the gearing's been changed to accommodate your sidecar...