I'm now using Marvel Mystery Oil in the gas, carefully mixed at 4 oz per 10 gallons. Now here's the test I conducted. About the time I got my Jeep 4 months ago I became interested in MMO due to good reviews here and elsewhere, so I started sporadically adding a little to gas now and then. Over time it became my perception that the fuel gauge went down faster without Marvel in the tank, so I started adding it consistently. I then decided I needed to do a more controlled test to see if I was wasting my money or not. Here's what I did. With MMO added to the gas, I ran three complete tanks of fuel. My routine is very consistent and is a mixture of city and highway commute driving. I refueled at that exact same Phillips 66 station, used the exact same pump, and filled the tank on the lowest flow setting and removed the nozzle as soon as it clicked off the first time, no top offs. I considered this the best way I would be able to get a consistent fill without variation. It has been beastly hot this whole period so AC use has also been consistent, as well as tire pressures monitored. All calculations were done using the trip odometer and actual fuel used. All three tanks with MMO netted 18.1 to 18.3 mpg. This is for my 04 Grand Cherokee 4.0 I-6 4wd. I then refilled the exact same way and ran a tank without MMO. I got 15.96 mpg. Nothing changed that I know of than the MMO being excluded. I didn't consider a single tank a conclusive sample so I ran another. That netted me 15.56 mpg. I wonder if a little MMO was still in the system the first time, as I dropped another .4 mpg on the second tank without it. At that point I'd seen enough so I refilled and added MMO. Here's the real sweet part, a full 20 gallon fill of gas only takes 8 oz of MMO which costs a whopping $1.33. At + 2.5 mpg on 20 gallons thats another 50 miles, or $10.22 worth of gas in these parts, so even after deducting the MMO I pocket almost $9 a tank. I'll take that deal anyday! Why does it work? I'm not really sure, MMO claims it improves combustion and lubricates the fuel system. My suspicion is that it's helping overcome that [censored] ethonal the govt is making them put in my gas. Note: MMO recommends 4 oz to 10 gallons and more is NOT better, in fact I plan to experiment with a little less on down the road. I don't know if it will work for you but it is for me and I'm now introducing it into my other vehicles. No I don't work for MMO or anything like that, I'm just trying to help my fellow BITOG'ers save a buck. I know a lot of you are already using MMO, but those are my results as best as I can document them.