Can I charge my iPad from a car USB?

Yes, but depending on how many watts it charges with, recharging may take longer than usual.

It will probably charge very very slowly, but it depends on how much power the car usb is providing.

And don't be surprised if you get the "Not Charging" label when connected.

The iPad 4 uses a 12W wall charger. The standard USB only provides 2.5W, so charging will be V...E...R...Y Slow.

 Cheers, Tom 😉

If your car has the 12v power socket ( used to be cigarette lighter socket )

You can buy a USB insert with one or two USB sockets delivering only power

I have a Scosche device which has 2 x USB sockets both delivering 2.1 Amps designed for iPad charging

I bought some bargain lighter socket adapter (might be Kensington ?) on Amazon with a 2.1 or 2.4 amps USB slot and it charges my retina iPad fine (I think it is also rated at 10watts ?). My car does not have a USB port built in, but the power socket adapters are cheap and work well.