[Christ]

Adding ANYTHING that is cheaper than gas, or even the same price, and doesn't REDUCE your MPG has increased your GAS MILEAGE.

I say this b/c:

Iif you're getting 30 MPG on gas only,
and you get 30MPG still, after adding 1 gallon of additive to a 10 gallon tank,
You are only using 9 gallons of gas, thus getting better mileage per gallon of gas.

Full gas = 30 MPG = 300 MPT = 30 MPG of gas.

Mixed = 30 MPG = 300 MPT = 300/9= 33.3 miles per gallon of gas.

You'd have to keep doing the math to figure out where it would be most economical to stop adding things to your gasoline, but if you add ANYTHING and don't lose mileage, you're still getting better efficiency... if the thing you add is cheaper than the gas you're using, you're saving money too.

Xylene is about $22 a gallon from the hardware store, hardly cost effective. (I only tried it b/c a 55 gallon drum can be had for ~$3/gallon, and if it provided ANY improvement, it would have been worth it with gas @ $3+/gallon.)