NO drop in mpg, just quieter running. I just did a 600 mile trip, running 75-80 and got 32 mpg, just like always. Ran 85-92 coming back, mpg dropped to 30, just like always. Found more GC when I got to my destination, and added to my stock. I only have a few oil changes of GC hoarded, but, that might last me nearly 30000 miles! By then, perhaps the GOLD will have proven it's mettle.