I did see an increase but it was hard to tell how much. Since I pulled a drop deck I only put it on the tractor. Also, unfortunately I put another fuel saving device on at the same time.
I think that unless you are going to drive A LOT of miles a year, anything that you spend much time on money on just isn't going to save you much. Unless, of course you just want to see if you can do it.
You will probably save as much or more by adjusting your driving habits.
You can crunch some numbers using, say 100,000 vs 20,000 miles a year and different percent of increase, and even different $ per gallon.
Another problem you will have is getting accurate usage data. In the truck when I drove 130,000 a year averages tended to even out somewhat. It makes a big difference, where, when and temperature of fuel you buy. Do a search on hot fuel, you get less.
FWIW if memory serves me right my yearly average was about 7. Highest I ever got was 13 headed east of of Denver, had a great tail wind. Of course going west on the same trip I only got 2