What will upgrading wheels do to the speedo

[r89vette]

Hey Guys, I have an 89 vette with 275/40-17 all around and I looking at picking up some C5 wheels. From what I can tell the backs on a C5 are 275/40-18 from the factory making about a 1 inch diameter difference from mine. Assuming what I found on the web is correct, that would make about 2 mph difference on the speedo. Can than computer be reprogrammed to take the bigger tire into account or do I just live with the speedo being off a couple of mph? Thanks for the help!