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.
Hopefully more apparent on single carriageway A-Roads, don't forget the extra fuel used to accelerate away from traffic lights, roundabouts and general stop-start driving.
Planning the route to include as much free-flowing or constant speed roads makes just as much difference as driving at either 50 or 60.
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