Range estimates are irrelevant since it will depend on where you are, what you are listening to, etc.
What I can say is that when you use a hand held scanner in your car, it's going to suffer. Surrounded by the metal body of the vehicle, and often tinted windows that contain metal film, defroster wires, etc, it's going to reduce the signal that enters the space.
Putting an antenna outside the car where it's in the clear will greatly improve coverage.
Avoid gimmick hobby grade antennas. Ideally something like the Larsen NMO-150-450-756 is a good choice that will likely outlast your car and scanner. Pay attention to where and how you mount it. Ideally you want a permanent NMO mount through the roof of the car. That provides the ideal ground plane and isn't blocked by parts of the car. Some don't want to drill holes in their car, and that's a shame, but I get it. Mag mounts can result in paint damage. Trunk lip mounts, etc. are all options, but also come with their own drawbacks.
But, yes, you'll see a considerable improvement in performance by getting the antenna outside the cabin. As for range, no one can really tell you that.
I have the antenna referenced and it is very good and should last forever. Using a magnet mount - very heavy - dead center in the roof of Explorer. Best omnidirectional placement. Mount toward rear of roof for directionally foreward.