This is an article at espn.com by Christina Kahrl. Here is the link
She points out that Gehrig did not have to face the good pitchers on the Yankees, and, of course, no black or Latin pitchers. But although those things are true, I wonder how much they would or should change our estimate of Gehrig. My guess is not much.
Take a look at the pitchers that Hank Aaron faced at Baseball Reference. Rank them by PAs. I did that recently and after a good chunk of them (the top 100 in PAs vs. Aaron) it looked like 13% of those PAs were against black/latin pitchers. That is the kind of thing that has to be taken into account in these discussions. If Ruth & Gehrig had to face about the same proportion of black/latin pitchers that guys today do (or post 1947), we need to put a number on it and then try to figure out how many HRs they lose from facing, on average, better pitchers. Remember, that Ruth & Gehrig would still hit some HRs against those guys.
But suppose that you did have them in about the same proportions that we have had them since 1947? I estimated about 10 years ago that would be about 15%. Then I estimated that it would have reduce Ruth's HRs by 5%. See
From 1928-31, Gehrig had a 1.040 OPS against the A's. The A's had 2 good lefties, Grove and Walberg and the A's led the AL in ERA+ 3 times with a 2nd. They led once in FIP ERA and were 2nd 3 times.
Gehrig did have an OPS of 1.109 overall in those 4 years. So he did not do quite as well against the A's. But remember, this was the best or 2nd best staff over those 4 years and he did not drop off by much and it was in about 380 PAs.
I also estimate that, if we take the Yankees pitchers out of the league averages for OBP & SLG, and then recalculate the league OBP & SLG, so that we can recalculate Gehrig's OPS+, I get it going down 3 points. If it falls from 179 to 176, he is still in 4th place. See