When quoting ERAs from 40 years ago you should be sure to give the league average ERA for comparison purposes. A 2.76 career ERA was great for when Koufax retired, but if a starter retired now with a 2.76 era it would be incredible. There are only two active starters in the major leagues with 1000 IP that have ERAs under 3. Both of them have higher ERA pluses than Koufax. Let's also not forget that Koufax pitched in Dodger Stadium for most of his career which is a pitcher's park, and that's adjusted for in the ERA+ I believe.