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.
How old are the cars you’re thinking of that don’t like going 20mph? Do you mean they are more twitchy and will accelerate too easily even in 3rd gear?
I retired my 2003 VW last year after 15 years of service (F fallen comrade) but it couldn’t do 20mph in 3rd, only in 2nd.
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.