Impossible to answer with absolute certainty, but if you are strictly talking sidearmers and no submariners, that eliminates all but one.
Only Mets righty Joe Smith throws a true sidearm pitch and he sits 89-92 with his fastball.
If you include the submariners such as Byung Hyun Kim, Minnesota's Pat Neshek among others, the average probably drops a bit. Submarine pitchers tend to rely more on deception than velocity and often rush it up to the plate in the 85-88 mph range.
89
88
The average fastball speed is 92 miles per hour. In 2003, Billy Wagner threw 25 pitches that were 100 miles per hour or faster.
75-80 mph
A baseball pros fastball is on average 92 mph
55-65
The average speed at a school with a good program is about 65-70 MPH.
College fastpitch pitchers average in the 70s. MLB players pitch in the 90s and occassionly top 100mph.
In travel team tournament play, about 55 miles per hour.
77 mph www.hardballtimes.com/main/article/fastball-slider-changeup-curveball-an-analysis/
Throw it harder
Aroldis Chapman threw a couple 105s
Bob Gibson has the 7th fastest fastball in major league baseball history. His fastest fastball was clocked at 96 miles per hour.
im not quite sure but I would say probably around 70s to mid high 80s im