Depending on your tire, that was around 34 or 35 mph.

I rode with CKAudio and Airxxxwolf today. We did loops around a park, where there was both a road and a path system around a lake. For the most part, we stuck to the road. At one point a pickup truck went by, so I sprinted to get in his draft. I was doing between 35 and 40 mph for several miles, with the guy in the truck checking his mirror every 15 seconds to see if he'd dropped me yet! On the next lap, I powered down one of the sweeping hills and hit 41 mph. These were only about 3% grades, not the huge grades that generate the *really* fun speeds.