I used millers diesel additive in my old escort td but i used it regurally over a period of time i did notice a difference and i use to get an extra 25 mpg to a tank full.But it wasn"t overnight!! As i owned it for 12 years..I did put a one shot redex petrol additve in a subaru i had did it twice over a month no difference.I think its a more long term thing that you do regurarly.