You'll be traveling 3% faster than your speedometer says you are. For example, at 60 mph on your speedo, you'll actually be going 62; about 31 when speedo says 30.
To recalibrate the speedo/odometer (yes, trip and odometer readings will be wrong, too; even a 3% error can add-up to a pretty inaccurate odometer over time...if you care about that sort of thing), you need to reprogram the computer. There are tools you can get, or you can have a stealership do it for you for like $60-70ish.
1983 Chevy El Camino
1994 Cadillac Eldorado ETC
1996 Jeep Cherokee Country