When engine is cold, rpm metre is higher (ranges from 1,000rpm to 1,600rpm, depending on car model) as all cars are electronically tuned to have higher rpm during cold start to warm up the engine.
When engine has reached optimium operating temperature, which is about 5 mins after you start your engine, rpm at idle should be 800rpm for optimium fuel economy.
If your engine is already warmed up and the rpm is above 800rpm at idle, it is drinking more petrol than necessary.