I was on crutchfield checking out speakers for my nissan frontier
and I got to thinking.... Which would be better, a 6.5 shallow, or a normal 5.25 of the same make and model.

What is the theory on this?

 

Stick w/ the 6.5's if you can. You'll get more range out of them.

 

Check the frequency responses of both speakers. The 6.5s may go a little lower. However, if you are installing them in a system with subs you may not need some of the lower frequencies since they will reproduced by the subs.

 

fyi crutchfields measurments have been known to be wrong