a few notes to help compare these sound readings:
Make sure you're using sound
pressure and not sound
power; the two are not directly comparable or interchangeable. Most simply stated, sound power is how much sound is being emitted by the source and isn't as useful without putting it through calculations and data based on distance from the source, sound damping of objects between the source and the listener, reflective surfaces behind the source, etc. Sound pressure is how much sound is being received by the listener and will better translate to what you hear as the vehicle occupant.
If you have the option to read an A-weighted sound pressure (often expressed in dBA instead of dB), do that. It's adjusted to different levels for each frequency band to account for the varying intensity that different frequencies are picked up by the human ear. The same sound pressure is perceived with different loudness based on the frequency, so 75dB at one pitch may be more or less offensive than the same at another pitch; using dBA helps a lot to correct for this.
https://en.wikipedia.org/wiki/A-weighting
Lastly, since decibels are measured on a logarithmic scale, they're not added together in the traditional fashion. For example, two identical 70dB sources added together measure 73dB. Adding a 70dB source to a 75dB source results in a total of 76dB. Adding a 59dB source to a 70dB source totals...70dB. Good info here:
https://www.engineeringtoolbox.com/adding-decibel-d_63.html
@anarce I think is the one that does this sort of stuff for a living, I'd bet he would have a lot to add (and may even have some corrections to my post).