It's actually more efficient to increase the amount of oxygen per gasoline. The only way to conventionally do that is something like a turbocharger. You provide more oxygen to burn, giving you better combustion ratios. Without the oxygen it doesn't matter how you use that gas, it simply won't burn. How do I know this? Because every gasoline car emits unburned gas. You could fine tune the amount of gas burned, though chemistry will tell you at the temperatures found in a car engine a 100% burn ratio for oxygen is near impossible (if not flat out impossible, I don't actually think there is a chemical reaction that will use 100% of the oxygen). So you still can't burn all the oxygen, which would mean the gas won't burn, so your still wasting gas. Eventually you'll get to a point where the gas won't ignite, it's how it works.
Simply put, dude was a con artist and a good one.