So I collected some data on my truck using this product.
I did the exact same drive of about 1000 miles a few weeks apart - one before the BG44K, one after. This is the same route that I take on my frequent trips from LA to San Jose via Lake Isabella, then back - same roads/speeds, same stopping points, same fuel stations each time. More or less the same weather.
On the trip after BG44K, I used a total of 0.4 gal
more than on the trip before the treatment, out of about 60 gal total. So essentially zero difference, maybe a slight decrease, but in the noise compared to all other factors.
What is more interesting is that on my drive, there are two distinct segments - one on slower, more windy roads, where I averaged about 60-70 mph and got 17.3 mpg, the other on I5, where I averaged about 76 mph (shhhh) and got 15.7 mpg. And this was consistent from trip to trip. So speed was by far the bigger impact on mpg than BG44K.
So at least for me, the BG44K was a waste of $38 (I bought a 2 pack with the bonus funnel). I guess I have a nice funnel now

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I'm not saying this stuff won't work for others, perhaps it will help engines with very dirty fuel systems. Although I am skeptical since today's detergent gasolines are pretty darn good at keeping things clean.
My suspicion (and I would be interested in counter examples, backed up by data), is that the mileage "improvements" seen above were based on insufficient mileage driven, or more importantly, at different speeds from before and after. Speed has a huge impact on highway mpg, and unless it is consistent from measurement to measurement, the comparison is likely to be skewed more by speed than anything else.