If you're not sure of your original tire size, check your GVWR/GAWR sticker in the driver's door pillar area. If you go smaller, watch your load range and tire load rating to make sure you don't exceed the tire's load capacity.
Having said that, the 245 will effectively make the final drive ratio lower (numerically higher) relative to your current 285 tire. This will improve towing performance and help your bed rail clearance (with appropriate hitch/pin box adjustments). Fuel mileage effects should be negligable after odometer correction is taken into account.
2016 Ram Longhorn 3500 Dually 4x4 CCLB, 385/900 Cummins, Aisin AS69RC, 4.10
2014.5 DRV Mobile Suites 38RSSA #6972
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