The VIAIR 400P is a 12-volt portable air compressor that clips straight to the truck battery and puts out enough air to handle full-size truck and trailer tires at highway pressures. We run ten tires between our Ford F-350 dually and our Brinkley Model Z 3515, and this is the tool that made checking and filling all of them a normal part of travel day instead of a chore we kept putting off. If you tow anything heavy, it's worth the money.
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Why we bought it
Six tires on the truck, four under the fifth wheel. That's ten tires that run close to their maximum load rating, and a tire that's 10 PSI low looks completely normal right up until the heat buildup ruins it. The fix is simple: check every tire, every travel day, and air up whatever drifted. The problem is that actually doing that with a cigarette-lighter pump takes half a morning, and doing it at a truck stop means maneuvering a 43-foot trailer next to an air hose that may or may not work.
We already carried a small jump-starter compressor in the truck, and it's fine for a car tire or a bike. It is not fine for a heavy trailer tire running at triple-digit pressures. So the excuse to skip the check was always right there. The 400P removed the excuse.
How we use it
The compressor got its first real workout during our undercarriage maintenance day, which we filmed for this maintenance video post. The rig had been parked for months, and when we went around with the gauge, several tires had drifted well below spec. The 400P brought a low trailer tire back to spec in a few minutes, then worked through the truck tires without breaking a sweat. It has an automatic function that shuts it off, so you're not standing there guessing.
The routine now is the same every travel day. Pop the hood, clip the leads to the truck battery, start the engine, and work around the rig one tire at a time. We check pressures first with an ETENWOLF T300 digital gauge, because a cheap stick gauge and a TPMS reading can disagree by several PSI, and at trailer weights that matters. Then the VIAIR tops off whatever is low. It has aired us up in campgrounds and parking lots more times than we can count, which is exactly the point: no hunting for a truck stop with a working air pump.
What to know before buying
A few honest notes from living with it. It runs off battery clamps, not a cigarette-lighter plug, so using it means opening the hood and having the engine running. That's the price of real airflow, but it does mean this is a pop-the-hood tool, not a glovebox gadget. Related: it needs a live battery, so it won't save you if the truck is dead. We still keep the small jump pack around for that.
It's also a kit, not a single gadget. Compressor, hose, and accessories ride in a carry bag that takes up real space, which is fine in a truck bed toolbox and less fine in a small car. And while the kit is rated for tires up to 35 inches, we treat its gauge as a ballpark and still verify final pressure with the T300.
Where to get it
We bought ours on Amazon, and it's the same kit we've been using since that maintenance day: VIAIR 400P portable compressor.
The compressor is one piece of a travel-day routine we've written up in full in our towing, hitching and setup gear roundup. Two other pieces of that routine have their own write-ups: the GEN-Y gooseneck hitch that connects the truck to the trailer, and the RV SnapPads that keep the rig planted once we're parked.