Best Winning Pct. Before the Slide Began

The following is the top ten career winning percentages for pitchers, minimum 150 wins, majority of career post-1900:

.692 211-94 Pedro Martinez (active)
.690 236-105 Whitey Ford
.680 300-141 Lefty Grove
.665 373-188 Christy Mathewson
.660 194-100 Sam Leever
.658 354-184 Roger Clemens
.655 165-87 Sandy Koufax
.651 170-91 Ron Guidry
.650 288-155 Randy Johnson (active)
.649 189-102 Lefty Gomez

But this is one of those stats (batting average is another) where your number can go DOWN from it's high. In fact, pitchers USUALLY win less often as they get near the end of their career. So I've compiled the top ten winning percentages, 150+ wins, at the end of a season in one's career (before it fell lower when later years are added in):

.720 216-84 Whitey Ford
.712 166-67 Pedro Martinez
.712 195-79 Lefty Grove
.694 154-68 Ron Guidry
.694 200-88 Grover Alexander
.688 170-77 Juan Marichal
.685 263-121 Christy Mathewson
.681 169-79 Mordecai Brown
.679 224-106 Randy Johnson
.667 328-164 Roger Clemens

You can see that a lot of guys at .667 or better well into their careers fail to sustain such a percentage on through the end of their careers.