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Your speedo will be out by that much. And you'll be revving that much higher, therefore wasting that much more gas. If you went the other way, you might have some FE gains.
Your cheapest bet is to pump the tires up a good % over recommended, within the max pressure printed on the sidewall of your tires. But you'll probably gain the most by switching to a Low Rolling Resistance tire, if they make them for trucks in your size.
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