This may be useful
ULTIMATE 3-WAY ARB Air Compressor Comparison {YOU WON'T BELIEVE THE RESULTS} - YouTube
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I have the dual air compressor and the 4 litre tank in my D4 in the 3rd row footwell area. The compressor on the left hand side and the tank on the right hand side. Can send through photos if it's of interest.
The intention of the tank is two-fold. 1) for air tools and 2) as a reservoir for an air horn to fit under/behind bull bar. Then there will always be air on tap ready to go instead of switching compressor on and waiting for lines to pressurize.
The car is not a 7 seater so when I said "rear" seats- it was rear for me - middle seats for those of you with 3 rows [smilebigeye]. seems to be a neat location- out of the way with enough clearance with a carefull install and a tidy secure wiring job.
Not so surprised by the tank/no tank comparison. You would need to be a reasonable size tank and at higher pressures (150-200psi) to really make a difference to tyre inflation. I would be interested to know what PSI the factory compressor maintains in the standard OE tank for the air suspension if anyone knows?
Cheers and thanks for the comments