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I'm guessing you 265/60r18's? I believe 275/6518's are a factory ford option so clearance wouldn't be an issue. But the 275 tire is almost a full 2" taller, so you will notice the speedo is not accurate, however with the right tuner that can be fixed. You MPG will be off to, but I would think not by much. The 275 will also ride better (if the two tires are the same rating, ei. both 8 ply tires), because of the added side wall. Handling will feel a little less thou... If it were me I'd swap the 275's and recalibrate for the tire size...
I've had all 3... Lariat came with 265/6018... Couldn't stand the tiny tires on the truck.. put on 275/65/18's.. MUCH better and the standard size the 4X4's come with... liked that much better... but then found the chrome 20's on Craigslist and bought for 1000... sold my old 18 set for 700... so got 20's for 300 bucks...
As far as the speedometer - on the 2015's and newer, Ford no longer allows or has software to re calibrate speedometer.. have to buy after market to do so...
Only thing that slows down the drive and feel is putting some heavy all terrain tires on - those michelins will be great and fine... the 275/55/20's are not as tall (barely) as the 275/65/18's by the way...