I think it's OK to use either gear.
I made up a chassis this weekend & used the 19T gear, but man there isn't much ground clearance!
On my Tomy track the gear would hit the rails if it slid enough.
I'm going to swap it for a 15T when I get a chance.
Another place to get Ultra G chassis is Mark Twain: https://www.hobby1.com/auto-world-thund ... assis.html
Sigh up for their emails, they have a lot of specials & get free shipping over 35 bucks too! :music-rockout: