It would have helped - had the OP posted the model number for the antenna and a description of what he wanted to do with it.

A mobile vertical antenna has no gain - gain is only achieved when you have gain in one direction and rejection in one or more directions.

The only measurable gain would be gain as compared to a dipole or gain over isotropic.

1/4 wave antenna's can sometimes produce a better signal locally, because the radiation pattern is spread out over a larger area.

It will give no distance gain - just local reception.]

Because it is all one radio - the reception length does not matter, just that it is resonant at X mhz - transmit.

Public service here all uses Larsen antenna's - especially the PA State Police, and they have very deep pockets.

Shakespeare also makes a decent mobile antenna.

You need a antenna analyzer or a Dip Meter to set to resonance.

The only thing the SWR meter can do is tell you what is happening in the feed line.