Will driving around town in 5th gear save fuel?

As we endure the £1+ per litre for unleaded petrol, I wondered if driving around in 5th gear on straight roads where the speed limit is 30mph would save fuel? If i drive at 30mph in 4th gear the rev counter says 2000rpm. However, if I drive at 30mph in 5th gear the rpm is 1250rpm. Will this save fuel if I do this?
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