Well, let's see, if you push 7psi, you are adding 50% more air, thus 50% more fuel. Running at 14psi, you're doubling the amount of air and fuel (and doubling engine output as well). Assuming you're at full boost all the time and that your gas mileage is always 30 mpg, you'd have 20 mpg at 7psi and 15 at 14psi. Mind you this is just theoretical, and won't actually reflect your gas mileage. Also, remember this is premium gas.