This question came up in a friend's letter to the editor in my hometown
newspaper. He had inquired with the state patrol about what the speed
limit was on county gravel roads; the response was that it's the same
as on the highways. The writer cited an incident in which someone was
flying down the gravel at at least 50 and hit a dog...and kept going,
right in front of witnesses.

I used to live on a gravel road, and anything faster than 35 on most of
them in northern Missouri is asking for trouble (dust, control loss).
Some locations, probably by county law, post a 35 speed limit. The
only gravel road I know of that has a posted speed limit higher than
that is AK 11 at 50.

I would imagine that enforcement would be rare as it would be under the
jusridiction of the sherriff's department.