Usually, car antennas aren't that terribly caring about antenna length as long as it grabs enough signal. They use short ones, collapsable ones, dipoles that are part of the windshield (windscreen), etc. Usually, at least in the old days, the FM section of the radio took what it got and did well with it. The AM section had a padder capacitor on the front of the radio under the tuning knob that was peaked for whatever antenna was supplied. These days, AM is thrown in on a car radio with some el cheapo circuitry just so it's available and they spend most of the design on the FM portion.
If the antenna length was that critical, then they'd mount a power antenna on the car and it would automatically adjust for antenna length according to the frequency selected on the radio.
Dean