My last ranked series game I faced seaver who had a 96 mph 4 seamer and a 76 mph circle change.
I don't recall a single pitcher in MLB history with a 20 mph difference between the fastball and changeup, likely because a grip itself is unable to account for that much difference in velocity without also slowing down the arm action. As far as I know Seaver in-game has the exact same pitching motion for both pitches.
Please consider velocity differential when creating legend cards in DD. Most of us want to play a realistic game and this just doesn't make any sense.