Air tank to inflate tires

I take my taco out in the dunes alot and most of the time I need to air down to 18lbs. Does anyone know how large of a spare air tank I might need to air up my tires to around 40lbs after I come out of the dunes? I usually air up at a nearby market but sometimes I can't do this and was wondering if I could just carry compressed air in a tank to fill up afterwards.

 

If you don't want an onboard compressor you need a Co2 tank. PowerTank makes them

https://powertank.com/

 

Warning: rounding ahead, so answers are approximate.

A 33" tire on a 17" rim with a 12" width has approximately 7500 cubic inches of volume. PV=PV, so to take a 7500 cubic inch volume and increase its pressure from 18 to 40 psi is 7500*22=165,000 (ignore the units for now). Assuming your tank is a 150psi max, and you don't want to go below 50 psi in the tank, because it'll take forever to fill up a 40psi tire with a 50 psi air source.... you have 100 psi to work with. 165000 divided 100 is 1,650 cubic inches of air. Convert that to gallons and you get just over 7 gallons. As you take air out of the tank it will cool it off, so you'll lose more pressure than what these numbers show. I wouldn't go smaller than 10 gallons at 150psi, and I would verify that in your driveway before you have to do it on the trail.

 

I bought one of these and it has worked out well. Clips to a 12V battery and for ~$140 I don't think you could get better quality. Check out the reviews as it looks like the top reviewer has your exact same use case and offers his feedback. Worth considering. Every time I bust this thing out at a gas station to top up my tires I get construction guys asking where I got it. Napa son.

I ran a VIAIR setup in my diesel rig years ago and had the 5 gallon tank and installed air chucks on both nerf bars - that was nice but a pain in the ass to install and regularly maintain. The only reason I would switch from my current setup is front and rear air lockers.