Using Lucas Fuel Treatment I actually recorded a DROP in MPG for that tank, did'nt run it in my next tank and I had an increase from average MPG.
So to test, I ran it every-other tank for about 7 or 8 fillups, sure as anything, when it's in the tank, you get worse FE, the next tank you run your mileage will increase, but only by about 1-2 MPG over average. There is no long term (2, 3, or 4 tanks after the Lucas was used) increase in FE.
If you add up all the pennies it takes to go 2 miles, and add up how much that 3oz. squirt of Lucas costs, it equals out to basically provide no money savings, or even cost you a few cents.
These weren't scientific tests, there was no control, and the routes and average speeds along with ambient temps were all different each tank... but none the less, convinced me enough that it isn't worth it.
I read an older article on treatments with my last daily driver. It talked about the amount or lack of treatment in the gas out of the pump. I decided to use Lucas on my old daily driver but only at about one forth of what it called for by the gallon. The first few tanks I didn’t see any changes, but then I started to see 2 to 4 MPG.
Watch what treatments you use and the mix per a gallon. If you are going to use something strong then it’s best to use it 2 tanks before an oil change so the stuff that gets into your oil gets changed out and dose not sit in the oil to long. Some of the treatments can leave a residue on your plugs.