Cam, I happen to be digging through my tool chest and came across a dB meter I apparently bought or got to review at some point. I pulled it out and tested it on the way to work in the Audi and measured 67-68dB at 80mph. 91.7dB max with stereo at level 22 out of 30. The P-Zero tires are extremely noisy and also just removed my rear mufflers. I wish I had found the meter a few days ago before i removed the mufflers, although it's barely any louder in the car at speed. When the SLC gets here (I go to mexico to test drive it while it's not registered), I'll relay how loud mine is for comparison.
I suspect your meter is using an A-weighted scale; I typically leave my meter in C since that's the mode I normally use when calibrating my theater room. I believe the A weighting gives you a value that's more relatable for the human ear whereas the C takes all sound energy for all frequencies and gives you a value, with very little adjustment.
In the office, with one conversation happening a few cubes over I'm measuring 35-40dB (A) and about 53dB (C). When using the A measurement sound energy is effectively "deleted" for frequencies most human ears can't hear (very low and very high). I just downloaded the "NIOSH sound level meter app" and I'm using my iPhone's internal mic for this A/B test right now. It's probably debatable which scale should be used for the purposes of this conversation but there are aspects of being in a car that can drive comfort that are based on frequency - for instance that very low frequency booming sound you can generate by cracking just one window open. If we were taking this measurement with the A rating selected I suspect it wouldn't show much energy - but throw it on C and you'll find it jacks the value up considerably.
My meter is the old analog Radio Shack unit that's decently calibrated and I've correlated it against the mic I use to calibrate my theater room - it's within 1dB for most practical frequencies (less accurate as you get deeper into sub-audible levels). I'm also holding the meter up, closer to shoulder level and to my right. The following info was measured with my Radio Shack meter, C weighting, slow setting.
My S5 is on relatively new (less than 1000 miles) Pilot 4S tires at about 35-38psi (I forget now) and I have a decent amount of suspension work (Bilstein B16 coilovers on comfort, solid end links, stiffer sway bars) - so possible some differences in our cars due to the suspension work.
On my S5 at high idle I'm getting 76-80dB sitting in the car, windows shut and radio off. At normal idle it comes down to ~74-76dB inside the garage and out in the open street.
During moderate acceleration I'm seeing ~100dB just before shifting. As with the Q5, sound levels seemed to be pretty independent of speed. I was measuring ~86-90 dB under light acceleration/cruising from 60-80mph. With my foot completely off the throttle, drive mode, 7th gear, cruising at 80mph I saw maybe ~2dB drop - so the variation in road and wind noise seems to drive more change than being lightly on the gas to maintain speed. That seems to match subjectively with what I'm hearing.
I'll try and repeat the measurements on my way home using the A scale.