Colder or hotter depending on the situation
may or may not improve your mileage.
From the NGK website:
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The spark plug firing end temperature must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called “Thermal Performance”, and is determined by the heat range selected.
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In nubbzcummins case.
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With colder plug it would hold 2 deg more timing pulling a hill resulting in more power and less downshifts to 5th on some slower inclines which causes real time mpg go down.
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The long term effect of running this plug has not been determined, being colder it
may not fully burn off deposits and require more frequent replacement or a cause long term reduction in fuel economy. The most common reason for changing heat ranges is engine modifications an example: raising the compression. A car that is burning oil
might benefit from hotter plug to reduce fouling.
Go here for a spark plug overview:
NGK Spark Plugs USA