I bought a 13" rubber job from KSaAuto on Amazon mostly due to their published VSWR plot over the FM band. I deal with a lot of antennas and range testing so this company sounded like they know a little something.
I just ran a test comparing the stock antenna to the 13" with stations in the low, mid and high end of the AM and FM band. On AM a really weak station on the stock antenna just goes away with the 13" on any part of the band. On FM the difference is much less with most weak stations on the stock antenna still there on the 13" but just weaker. In one case at the upper end of the FM band a weak station on the stock antenna was not useable on the 13"
I used to work on car stereos years ago (70s and 80s) and most radios had a tunable capacitor inside to optimize AM reception since the antenna and its interface, cables, etc, are very critical. The capacitor was accessible on the rear panel of the radio or through the cassette door. Years ago I found you could go from a large to small antenna and after peaking the capacitor there was really no difference. Every car stereo installer knew this and when installing a new radio or antenna you had to perform the adjustment.