Tire Size VS MPH

I was wondering if anyone knows if you can adjust your speedometer? I put on 30" tires and now my speedometer is off, when I am showing 60 MPH, I am going about 67 MPH. Not that big of a deal, but I do alot of long distance driving and when going on 300 mile trips my odometer is not matching my directions I am reading off of. If not I guess I just need to figure the math. Thanks, Rob.