Speedometer Recalibration

[CWOFOR]

I have a '17 Silverado WT with 20" wheels and 275/55R20 Conti's (came with the truck). I'm looking to go to either Goodyear Duratrac or General Grabber AT2, in 275/65R20. Both tires will increase the overall diameter by 2.1". I talked to my service rep about recalibration of the speedometer and after his research of the service manual, he tells me that there are no provisions for recalibration contained in the manual. Is this true? I know that 2.1" may not seem like a lot or may not be off by much; however, it could make the difference between a ticket or not (depending on the cop). I also find it hard to believe that GM wouldn't put in some provision for this. Can one of you computer guru's please advise a dumb old jarhead on this issue?