Seems there were wide variations in the early mechanisms for measuring the speed of pitches. According to the link below, he was indeed measured at 83 in the first measurements of the velocity of any pitcher, but he was also measured at 91 mph in another test and at 99 in yet another.
http://scoutee.co/the-historic-quest-for-speed-in-baseball/
The 83 mph test measured the total travel time of a pitch that Johnson, and another pitcher, threw in street shoes aiming at a slot 60 feet away. Flat concrete floor.
Pitches now are measured at release-point, which is much higher. MLB Gameday will show release speed and total speed. After reading a discussion of Johnson's speed, I started looking at total speed on Gameday to see what release speed gave mid-80s. Johnson probably threw mid-90s if measured the way modern pitchers are measured.
In other discussions, someone observed that pitchers from Johnson's era threw harder to the really good batters. It was estimated that Johnson and Smoky Joe Wood could touch 100 if pitching to Ty Cobb, but only needed 85 or 90 mph for others.
(In the late 1930's, Johnson admitted that he threw harder than Bob Feller, and Feller agreed. Read Hank Thomas's biography of Walter Johnson to understand why someone had to provoke Johnson to "admit" his comparison to Feller. It's great off-season reading.)
