GVW with wdh and without

you'll likely see a noticeable gain in mileage at 55ish vs. 65ish. On my duramax towing at 55 vs 65 gives me another 1.5mpg on either my outback or my cargo trailer. Above 65 mpg goes down in a big hurry.

wind resistance effect on mileage (which is the dominant drag when pulling a trailer at highway speeds) increase as the square of the speed. same effect for engine rpm and drag from a rotating tire. those three factors can add up in a hurry and send mileage down noticeably.