Aloha Paige ( @pyerxa ),
Thanks for your question. This is an important consideration for teachers and students designing small cars for classroom use and competitions like the Solar Sprint. I find that Tamiya Gearboxes are highly adaptable for multiple projects, including designing and building Chem-E-Cars ( https://www.aiche.org/community/students/chem-e-car ). This link has a good example of their variety: https://www.pololu.com/category/34/tamiya-motors-and-gearboxes
Personally, I like the Tamiya 72005 6-Speed Gearbox Kit ( https://www.pololu.com/product/74 ) for its multiple gear ratios. Have you had success with any other gearboxes?
Please let us know what you come up with!
Edwin ( @edwinjcolon )