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.
The average fastball speed is 92 miles per hour. In 2003, Billy Wagner threw 25 pitches that were 100 miles per hour or faster.
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.
75-80 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.
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
60mph