My 2012 C5 (UCE) with Inder sidecar will comfortably hold 60 mph. It tops out at 70 max. It has held 65-70 with a 200 lb passenger in a dash to get out of stormy weather.
Mine also has a brake in the sidecar wheel tied to the cycle's rear brake. This helps the rig to stop much straighter - no "push left" by the sidecar during braking.
I have my rig for sale - link to CL ad posted in the "Wanted" board as looking for a new home and posted on CycleTrader. I probably have a higher asking price, but am open to reasonable offers.