I just ran across an interesting chart. 1934 speed limits. Thirteen states were 45 mph, Eight states were 40, Seven states 35, One state (ND) was 50, DC was 22. Others reasonable and prudent.
In the late 40's the California speed limit was mostly 55 on highways, 45 for trucks. The road between L.A. and Las Vegas was posted as that but the CHP encouraged truckers to go 55 if they could! The reason? The road was filled with gulleys so cars could be hidden from sight at times. The CHP didn't want a car to suddenly come upon a truck doing only 45. Amazing but true, my Dad was a trucker back then and had a lot of neat stories to tell.
Kansas did not have a speed limit until mid 1957,reasonable & prudent. Many of the Northern States (ND, SD, WY, Montana) I think were all reasonable & prudent until the 55 mph came in in the 70's.
Utah's speed limit was 60 mph until the war started. My Dad's speed limit in our '31 Chevy was 40. As stated above, during the war it was 35, if you could get any gas.