There's no worse feeling than having the extinguisher go empty when you still have fire to deal with. Buy the biggest one you can fit in the car. I recommend one with a flexible hose because if you are fighting an engine fire from beneath, some extinguishers won't operate upside down. You keep the extinguisher more or less upright and use the hose to snake around the chassis parts to spray up into the engine compartment. Hose is also good when you pop the hood open only a couple of inches, then insert hose as far as you can over the top of the radiator. For mounting mine inside my pickup I use two Davis Super Quick Fist strap/mounts. It's a boating product good for holding cylindrical objects like oxygen tanks and fire extinguishers. (Note: I have no affiliation to the company, it's just a great product!) One last thing to keep in mind is protecting the squeeze handle and pull ring from other objects and people moving around inside the car. Wouldn't want the extinguisher to go off inside the car while driving.