advice
 

Last CB I had was in a 1963 Corvair. Traveling between VT., MA, IL, & NC i felt it prudent to get a CB. I'm going to install a Cobra 75 WX ST in a 2017 Chevy Colorado (no real room for anything else). My question is best antenna and placement.
Thanks
Gerry

A four foot, tunable antenna like this would do you just fine ...

https://www.amazon.com/Wilson-305-4FD-Silver-Load-Antenna/dp/B005ALD33K/ref=sr_1_fkmr0_1?ie=UTF8&qid=1519987291&sr=8-1-fkmr0&keywords=cb+antenna+wilson+4%27+turntable
Avoid shorter antennas because they are more difficult to tune, and suffer reduced range. Many other brands, but I avoid Firestik just because they promote that nonsense about the length of the antenna cable. That is pure BS on their part, despite their "theoretical discussion" which illustrates that the writer has no clue about velocity factor. I'm a retired USAF Chief Master Sergeant with over a quarter century working on military aircraft radio systems, including a 15 years engineering comm and computer systems for DOD C4I. Bottom line: The length of your antenna cable does not matter a hoot, except that you want it as short as possible. It is not a resonant element in your antenna system (at least it better not be!), so ignore anything that says 18' is the correct length.

As for placement: On the center of the roof. If you don't want to drill a hole, or need to remove to get into garages, consider a magnetic mount.

BTW, just so you know: This is a forum for owners of Jeep JKs--Wrangler model years 2007-2018.