.
Might not be what you want, but I believe the Banks iDash lets you calibrate the speedometer and it'll show your corrected speed all the time. They also have some really nice "pods" to mount the iDash to the driver's side A-pillar (right below the grab handle).
.
I have nothing to do with Banks, except I'm a satisfied customer. The quality and usefulness of their products can't be beat - they do hard-core engineering and meet all state & federal regulations, no hanky-panky with Banks.
.
Banks iDash: SuperGauge & DataMonster
.