In short, I put two bottles into my car and the mileage went up about 10%, which I am very happy with.Long version:I have a 2004 Infiniti G35 Coupe 6 Speed (Manual) that now has about 76000 miles on it. The car has some modifications (cool air intake, lightweight pulleys, lightweight flywheel) that could affect fuel mileage, but theoretically they shouldn't (or at least not much). All maintenance is done on time - car is well taken care of.Either way, my car is rated to 19/26 city/hwy, and I drive a combination of approximately 35%/65% city/hwy (this is actually conservative, the highway distance is probably more), but was only managing 17.5-18mpg per tank on average. I never expect to get the full EPA ratings since I drive faster than 65 on the highway and don't have a feather light foot when driving around town, but I didn't expect it to be THAT bad.So I was reading around on the multiple car forums I'm on, and was recommended this stuff. So I went ahead and bought 3 bottles of this stuff. I put one in our truck (2009 Dodge Ram 1500 w/ Hemi) and 2 bottles in my G35 (I put one in when I filled up, and the second the next time I filled up).With my driving style being the same, my mileage has gone up now to about 19.5-20mpg, which is a pretty solid 10% increase. I held off on posting but I've put at least 20 tanks through my car since I used this stuff and the mileage increase is holding steady (if not getting better). As far as I know I have not done anything else differently that may skew my results.I haven't been monitoring the truck's mileage and since it gets driven by many different people I can't comment on the difference on that vehicle, but so far it's made a difference on my car. I'll probably end up throwing a bottle or two of this stuff into my car every 5-10K miles. Before anyone argues that you spend more money that way, at the mileage increase I got, I'm saving nearly 28 gallons of fuel every 5000 miles. Gas costs about 3.25 a gallon where I live (car takes premium) so that's over 90 bucks worth saved for spending 10-20 bucks on this stuff. Seems logical to me.