The loudest car we ever built at my shop could hold a steady 158 db, and once it peaked over 160. This was over 4 years ago, without any "Wells Fargo Truck" windshield or other unusual body bracing. This was in a full-size Suburban, with 12 of the old-style JBL GTI 12 inch subs in 6 slot-loaded enclosures. Each sub was powered by a Rockford Power 500, bridged mono. Three more Power 500's brought up the rest of the sound, with various horns and mid-tweet combos. During regular listening at a usable volume, this actually sounded very good.
I was actually sitting in the front seat when we hit the peak. The RTA we were using to tune it only went to 160, and I was wearing hearing protection to the tune of the roll-up inserts covered by headphones. We held a constant 158 for about two minutes, and my partner signaled to go ahead just one more notch. We watched the numbers hit 159.2, then .6, then "OVER" and we called it good. Only when I shut it off and opened the door did I realize how much trouble I was having BREATHING!
If anyone's interested as to how we juiced that many amps, this rig had a custom alternator bracket holding 2 dual-250 amp alternators and custom rectifier setup, charging 8 Optima's and 15 Farads worth of caps (remember, this was still back when we only had the can-style capacitors available).