I honestly dont know. I dont have a bike computer yet. I can tell you that this past saturday on the Mt Vernon trail here in NoVA I was pacing autos on the GW Parkway, and passing a few. Speed limit on that road which paralells the Mt Vernon Trail is 35MPH. No one ever really goes 35MPH on that road though, its more like 45MPH. Ive driven it many many times, and if you go 35, you are being passed like you are barely moving.
That said, for a good minute I kept pace w/ most cars, and even passed a few. Fairly large hill that drops you steep at first then the incline gradually flattens more, but its still quite a drawn out downhill.
So im going to GUESS 40MPH