15.5 inch would be the best choice.
FM is tuned for a 31 inch car antenna. That's called a 1/4 wave. There's no room for a 124 inch full wave antenna. That would be the best reception.
Deviating from the tuned length will severely degrade reception.
A 15.5 inch antenna will be a 1/8 wave and will work much better than either a 10 or 18 inch rod.
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