In a notebook, starting from the front; I track every fill up, truck mileage, gallons, cost, and date. I also note if I was towing that tank or not.
In that same notebook, starting from the back, I track each maintenance performed, mileage on the truck, and date performed.
No other mods have been performed since the install of the pulley (unless you count a black headliner as a mod that would affect MPG).
So, in summation - mathematics, is how I am determining the delta, and can correlate it directly to the point of installation of the under-drive pulley & damper.
Correlation isn't causation; sure. However, I'll present a good case it is the cause. No mods, no change in route, driving, place of work, route, etc. Same tires, even, until 100 miles ago.
Installed at 22k miles, currently at 50k.