A few years ago, I bought a new gas can. I noticed it doesn't have a ventilation hole, but I figured there was some magic in the bizarre spout design that made ventilation unnecessary. I figured wrong. It takes several minutes to empty two gallons into a vehicle. I got tired of it today.
Step 1: Acquire useless gas can.
Step 2: Drill a small pilot hole in the handle, somewhere that won't leak when the can is full or in use.
Step 3: Plug said hole with a thumscrew, wing bolt or other suitable, secure device.
Loosely illustrated in attached photos. This works much better, but 1) might not be as safe for transporting gasoline, 2) might get you in trouble with some all-seeing government agency. As always: We're not responsible for problems caused by people who try this at home.