Antenna Help

I've got the K-40 Heli Coil Monkey Made wanna be antenna. My company does not want these large antennas on the mirrors so I mounted it on the back of the sleeper like some of the UPS trucks do. I try to tune it & the best I could get was 1.8 I went to try & re-tune & could only get 2.2. Even if I could get it below 1.5 would it perform good at all being behind the cab? The coil is about 6" away from the sleeper & about 6" below the top of the sleeper. At least 2/3 of the whip are above the cab. I mostly pull a tall side hopper (12'6" tall) or a short van trailer same height.
Thanks, Jason

 

Yeah it looks good back there but I figured it wouldn't perform well. I tried it with the special mount that that screws into a mirror mount for Volvo's but at certain speeds it resonated & sounded like I had a bearing going out.Maybe if I tilted it forward the wind wouldn't make it vibrate. Anyway thanks for the response.