Do you have any radar speed signs? Those things seem to be pretty damn accurate. They usually have them around school zones or in residential neighborhoods that they don't want people to speed on. I'd drive by that signs at different speeds and write down what ur dash says and what the sign says. The signs look like this.
I've seen signs that are 55 mph ones. This was the only good Internet pic I could find lol
Engine/Drivetrain: SCT iTSX with BAMA 91 Race Tune, Airaid CAI, JBA O/R H-Pipe, SLP Power-Flo Catback, Exedy Stage 2 Clutch, Exedy Lightweight Flywheel
Suspension: H&R SuperSport Springs, KYB Shocks & Struts