Vehicle noise is a mess of confusion, but:
noise is the larger of:
- speed in mph / 5
- engine power in kW / 40 * ( 3 + load * engine_noise_factor ) + 5
where load varies from turn to turn but is between 0 and 1 and is usually 1/accel for a cruising vehicle, and diesel engine’s noise_factor is 15.
A diesel v8 is 242 kW, so you should be at the higher of 9 or 6 * ( 3 + .01 * 15 ) + 5, or about 23. A muffler will cut the engine term down by 40%, so I’d expect you to beg getting around 13-14 noise if you have a muffler.
How much noise should a v8 make at 80 km/hr?