Dictionary.com
definitions

high gear

A state of maximum activity, energy, or force. For example, His mind was in high gear as he studied for the medical exam, or The political campaign is finally moving into high gear. This expression alludes to the high gear of an engine transmission, used at the fastest speeds.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.

Examples from the Web for high gear

Historical Examples of high gear