I have 15 Lariat that came stock with 18" wheels. I am looking to get a set of Fuel Revolver wheels in April. I want to go 20" on this wheel set and get snow tires for the 18" set. My question is.....

Will moving from the 18" to 20" thinking 275/55/20 affect my speedometer and/or mileage registering on the odometer?

 

With that tire size it has an almost identical OD and width to the stock 18's I had. So the answer is no. Wheel diameter doesn't change that.
https://tire-calc.com/#