Choosing an inflator

The 12 V one cannot be flat

It has no battery. Plug it into the car’s 12 V socket and it runs off the car, which means it works the day you need it whether or not you have thought about it since you bought it.

That is a bigger advantage than the specification suggests, because a tool kept for emergencies is a tool nobody maintains. The failure mode of a rechargeable device in a boot is that it is empty when the emergency arrives.

The cordless one does not need the car

The trade is that it goes where the car is not: a bicycle in a shed, a wheelbarrow tyre, a football in a park, an air bed at a campsite. It also has the better specification — 150 PSI, a preset you can dial in, and an automatic stop — and it will charge a phone.

And it has to be charged. Three or four times a year is enough, but it does have to happen.

The thing neither of them does

Neither repairs a puncture. If a tyre is losing air faster than a pump can put it back, an inflator buys you the distance to somewhere safe and nothing more. Both are for a slow loss and for the pressure that quietly drops over a cold month.

Pressure, briefly

Use the figure on the plate inside the driver’s door, not the maximum on the pump. A car tyre is usually somewhere near 32 PSI; 150 is the compressor’s ceiling, not a target. Under-inflation costs fuel and wears the shoulders of the tyre; over-inflation wears the middle and makes the ride worse.

So, which?

If it will live in the boot and be forgotten, take the 12 V one. If it will live in a garage and get used for bicycles and sports gear as well as the car, take the cordless one. If you genuinely cannot decide, the 12 V one is the safer default, because the way it fails is that you have to reach the car.