How would a Hustler RM-40-S with the 54" high aluminum mast (White vinyl coating) compare to the Tarheel(s) in your modeling? Given every thing else is the same. (Same vehicle, same mount, same mounting location.)
The RM-40-S inductor Q is probably close to the Q of the Little Tarheel II. As Alan points out the Super resonators have more loss than the standard resonators. With a 54" mast the Hustler antenna length is 103 inches.
Assigning an inductor Q of 100 in the bumper mount configuration the simulated free-space gain is -9 dBi. Compare this to the Tarheel 100A bumper mounted having a gain of +1 dBi.
How about mounted on the side halfway up the vehicle? With a Q of 100 the gain is -8 dBi.