This is just my observation from my own experiments. There has been a great deal of debate on the accuracy of the speedo (other speedo's) and GPS. Some don't trust the TW speedo as being accurate and believe in the GPS speeds. To be on the safe side, I think everyone should check their speedo vs. time, and then verify this with the GPS one uses. I think its possible GPS's may read differently too.
I'm always messing around timing my vehicles at different speeds to check for the vehicles speedo accuracy. All of my vehicles speedo's read correctly (relatively so, meaning if they are off, it is so minut I can't tell) i.e. 1 mile, 60 seconds, 60mph and so forth for the other speeds.
So, with the GPS I have, it reads 2mph slower than what the vehicles speedo says. 60mph on vehicle speedo, 58 on GPS. This 2mph differential happens on every one of my vehicles (F250, Jeep Liberty, and the TW). So in my case, I trust my TW speedo at the various speeds.
Now for my own top speed...on my 1% ride I took a while back taking pictures of the Old Ft. Boise Cemetery, I had and held my TW with stock gearing at 60mph and had throttle to spare. So, I suppose I could have gone faster, but I'm no speed demon. I'm nearly 50 and have never had a speeding ticket! Not that I don't speed, the man just ain't caught me!
