I ran an Overpass Turbo query for maxspeed=75 for the highways in this changeset, with the only exception being the railway segment in Michigan. Speeds seem reasonable for the rural states these roads are located. I do know for a fact such speed limits are found in Texas. In any case, mph should be used throughout.

I noticed all the roads in the Ward Orchards neighborhood of Bloomfield Township have been retagged from surface=compacted to surface=unpaved.

While they are technically unpaved, compacted is the more specific tag to accurately describe them, they were fine as is. Why were they changed?

Thank you, good catch! I fixed it in Changeset #135616926 just now.