If you are running a Holley try going down to a lower number power valve (say from 8.5 to 6.5). Vacuum conditions could be causing it to open slightly when you don't need it, this will make a big difference in fuel economy. Other carbs have similar adjustments you can make to them to change the power valve opening, you didn't say what you had.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo