i have tried all near where i am at, they are quite different

test is simple - fill up, run and calculate mpg between fillups.  if you have an onboard average mpg, you can use that too.

the funny thing is - shell is poor too.

the best in this area is rotten rabbies.  i typically get 35-36 mpg with it while 32-33 on costco and 33-34 on shell.