I put together a makeshift spoiler on the CR-V, small highway trip gave 30MPG at 72MPH cruise control, but still have to do a real drive to measure gain.
Edit: Today i got 27MPG at 73MPH cruise ctl, so not as good, mod impact would be negligible then.
tank with over-sized fins came out to 36.6mpg. I was caught in traffic getting into a high school graduation and got 20mpg on the first gallon of tank. I'd say the fins at least are not hurting so far. Coming home from the gas station ( 9.6miles) I got 49mpg with some EOC.