which is more accurate? scanguage or speedometer
 

my scanguage reads 2 mph lower then the speedometer on the truck. and it is calibrated correctly. which do i trust?

hmm, im not sure. 2MPH off isnt really that much, your speedo was probably off that much from the factory. if you have a GPS id check them with that.

I'd GPS verify and then go from there

Get a GPS like a Garmin. set the cruise at 70 on the garmin. then adjust the speedo on the scangauge to match the Garmin with the +- settings

If you have an iphone, there is an app that uses the accelerometer to measure speed.

That's the way I did mine. Rechecked it a while later and it's still good.

speedo's come from the factory a little slow so i would never trust that as a source of your "true" speed

Speedo is purposely slow to prevent liability issues on the oems part. scangauge is correct.

I set the factory speedometer to be accurate according to gps, using my programmer.