Depends on the road. Its not as simple as just the rpm.
This ^^^
Have the car on 'instant MPG' on the computer and keep and eye on that. Its more about gentle throttle, keeping the boost to a minimal.
Rich has nailed it there.
Feathering the throttle and keeping the revs low as poss without labouring the engine.
planning ahead and coasting down to junctions (with the car still in gear) saves the most fuel so anticipate corners and hills and either ease off to save braking or gently accelerate to gain speed ready for an up hill.
The less you have to use your brakes, the better your doing