Here's what I do: The 20 oz bottle clearly states it treats 'up to' 20 gallons (12 gal for the smalle 12 oz bottle). I'll pour 2 bottles in and fill it up (Use the plastic funnel as suggested). That's roughly 36 gallons total (and 18 gal/bottle average). I've done this three times now at each approx 10k mile interval, then try and take it out on a nice hot long drive at least once during that tank to get everything good 'n clean. It may just be psychological, but it seems to run quieter and smoother after that, and I usually get 1-2mpg over my recent average to boot (though that may be the highway miles talking).