The RPM is always dead on, but depending on your year, your car's transmission has an electronic or mechanical speedo. If it's mechanical, you could just put in the proper speedo tooth to multiply the revs/speed or get a SpeedCal box to do it. On the electronic ones, it's as simple as tossing it in a tuner and putting in the proper value for revs per mile, the number of tire revolutions per mile. This is also something that needs to be recalibrated if you change your tire height/aspect ratio as that changes the overall size of the tire.