Fuel consumption, expressed as gals/mile or litres/100km, is roughly proportional to the square of the speed.
Cruising at 60mph will give an mpg figure of about 70% of the value cruising at 50 - for example 21mpg at 60, 30mpg at 50.
Your overall consumption wouldn't see such a marked difference as the increased consumption during acceleration would apply to both types of road giving closer overall figures.