that is not an issue, the cruise control, ha ha ha. since this is an old mail jeep, whoever originally ordered it for their rural route skipped getting cruise control. but you are right, not using cruise on other vehicles makes for better gas mileage because of the same reason you spoke of, shift points.
i didn't try going up that much higher in speeds but did notice that at a few points where i had to get up to 75mph, which raised the rpms upto almost 2500, i think, did bring us down to 15mpg. my son drove it doing 80mph the whole way from colorado springs to lubbock and he said he always gets in the 12-14mpg range.