I wasn’t going to put words into your mouth that’s fairly accurate I'd say.
I measured the coach ride to London early this month at around the same value, I wrote down a figure on paper I did on small notepad, but I have several notepads and it looks like I need to fire my cleaning lady, because its mess around here.
The faster you drive the louder the sound pressure!
Also masking is your enemy on the road. You’ve got vibration that will in someway translate to sound pressure, the engine motor sound the air rushing around the vehicle and other vehicles as well all adding up to one miserable noise background level.
You might as well have cheap monaural sound system in the car or truck and wait until you get home where the noise background level will be considerably less.
You could ware headphones with ear defenders to block out as much noise background level as possible, but that would be illegal and you could end up getting nicked by the Smokey.