Regularly up to 43 on my morning commute, hoping the light at the bottom doesn't turn red. :-) 45 on the tandem on the same hill, still accelerating when we hit the bottom. 46 on the tandem is the max, descending into the Missouri River valley in western North Dakota. I think my max overall has been around 50 on a hill in the north unit of the Theodore Roosevelt National Park in western North Dakota. My mechanical speedometer of the day was printed only to 40 mph, so the 50 was an extrapolation on how far past 40 the needle went.