An air compressor is necessary for airing up after hitting a trail, but it can also be useful for helping people with a flat tire. If a tire has a puncture like a nail, the leak might be slow enough that you can inflate the tire and drive to a tire shop before it goes flat again. Additionally, it is common for a spare donut tire to be under inflated, making it useless. I have two compressors and I can recommend both of them. My battery powered Ryobi inflator will automatically stop when it reaches a target pressure. It also has a high volume inflator that's ideal for air mattresses. I also have a Viar P88. It is powered by clamping directly to the battery terminals. It has a duty cycle that is sufficient for Subaru sized tires, but Viar also make a xxx model for larger tires.