Even with the caveats about limited data and untangling causation and correlation, the statistics are striking: the first year of a scheme in Wales where the speed limit on urban roads was lowered to 20mph resulted in about 100 fewer people killed or seriously injured.
For what it’s worth, I wouldn’t be in 3rd gear at 20mph in a modern turbocharged car.
For my car, 20mph is roughly 2500pm in 2nd and 1700rpm in 3rd. I don’t like to live below 2000rpm because I don’t get spooled up that low.