If your rear is the only thing hurting during the ride, then its possible that the new seat will help.
If you are getting the 260 miles out of the full tank and filling up when on fumes, you're getting about 54 MPG. Depending on the speed limit of that road that may be what you can get unless you follow Vic's advice above. Otherwise the only thing I know of to improve mileage is to reduce speed. But I get in the high 40's due to my roads all being 70 MPH or more.
Sorry can't help specifically since I've not had a sore butt when riding. Hopefully someone else can provide you some insight.
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