Thanks for the info. I check my speedo periodically. The rig is 3 years old, leased from penske, and the speedo(analog) has always read 2mph faster than actual road speed. It's a 10 speed which I keep in 9th(max 59mph on a level surface) while in CA. I was on a slight downgrade with a light load, it is possible it gained one mph, or so, coming down the hill, but no more without either a significant load or grade, and certainly not 6. When his headlights came on I looked at the speedo and it read 61(2 over actual). I did calibrate the speedo once into AZ where there are regular mile markers: 5 miles in 5min&5sec with speedo reading 61(59 actual). He seemed like a nice enough officer, didn't even ask me for my log book, so I suppose he was being honest, guess I'll have to challenge his competency. Again, thanks for the info, I don't have much experience with citations as this in my second ticket in 25 years as a professional.