http://www.fueleconomy.gov/
A 2004 Durango 4WD with 5.7L is expected to get 12 city and 16 highway. You were exceeding EPA before your additive. 19 to 25 is a 30% increase.
If the additive has made a permanent change then the improved MPG will remain after discontinuing its use and the original baseline is not necessary, good for you but not for the additive seller. If the additive requires continuous treat for the gain then allowing it to dilute away for a few tanks will allow you obtain your original baseline.
Other than the generic nail polish chemicals Acetone and Methyl Acetate, Googling for "burn rate modifier" predominately produced results on one particular product resembling snake oil you've mentioned elsewhere. SXFT (XFT) is quite expensive. 20 Gallons of fuel at $2.50 per gallon with 1/4 ounce of SXFT for $4 at 24 MPG is $54 for 480 miles. The $4 will buy me 1.6 extra gallons. 21.6 gallons of fuel at 19 MPG will take me 411 miles. The remaining 69 miles requires $9 for 3.6 more gallons for a total cost of $63. The product is so expensive that a 30% mileage improvement produces only a 15% savings. The largest testimonial I can find is a 15% MPG improvement. I'll let you do the math on that one.
Try to think of a petroleum product that is $1200 per gallon. I can't think of any so there's a lot of profit in the mix, so much that ROI is slim to none. You should spend some time with other chemicals in hopes of finding a cheaper alternative. RXP Gas Kicker is a similar price and available at Autozone and other retailers, also not worth it even if it does work.
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