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245/40R18 are giant tires/rims. You would likely see a larger benefit going to 15" or 16" rims and narrower tires. 245/40R18 could be replaced with, say, 215/65R15 tires. 1.2% taller gearing, but more importantly a far lighter and more eco setup.
Wider tires almost never help economy unless you are comparing a wider, low rolling resistance tire to a narrower, non-eco tire. The heavier weight of a taller tire may hurt economy in the long run unless you do only highway driving.