What is average MLB fastball speed?

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.