As long as the actual tire diameter doesn't change.... you won't need to do that.
Remember, the speedo reads about 3-miles per hour faster than you're actually traveling, so there's that margin.
If you have a 55 series tire on an 18inch wheel (stock) and move up to a 20 inch wheel with a 50 or 45 series tire, it won't have a major impact, in any, on your speedo.
Am I correct here???
2006 V6, Campy White, VIN 46xxx, Anthracite w/ Sound 1 and Xenon Headlight Upgrades, Monster Mats, 3M Window Tint, Euro Headlight Switch, Gorilla Gear Cargo Mat, Clear Bra, VW Tow Pkg, VW V8 Style Exhaust Tips, 18' Manhattans & Silver Racks
2013 Audi allroad: Ibis White with Layered Oak Inlays