You will hardly notice the change. the main factors determining highway mileage are weight, rolling resistance and air resistnce. none of these change at 70 mph your rpm changes slightly but with the increased gear mechanical advantage increases therefor less cylinder pressure is needed to maintain 70 mph. But it is spinning slightly faster but you will not need to give it 10% more throttle to get there.

Your in town mileage will not change to any degree at all.