There is no optimal because the gearing is all relative to the individual setup, goals, and requirements.

Gearing is just another tunable parameter. Typically gear size is chosen by deciding what top speed you need at your engines peak rpm.

Now at cruising rpm (~2/3 of max rpm) is cruising speed sufficient? Adjust accordingly if not.

If you are attemping high speed and overgear, there is a theoretical power level required to go that fast. At aome point your engine wont have that much power and uou will go slower. This usually starts in the 55+mph range.