Speedometer Calibration

My Taco speedometer is slightly off and indicates I am traveling faster than I actually am.

Here are my readings:
Speedometer = 60 MPH
GPS (used two different models) = 57 MPH
OBD II Tool = 57 MPH

The vehicles computer digital MHP value does match both GPS units.
It also appears the speedometer reading is off because the vehicle computer analog output needs to be calibrated to match the speedometer inaccuracies.

Is there a way to calibrate the vehicle computer analog output to the speedometer?

And I asked this question in this manner because the typical answer I see is to change the tire size but that would then make the vehicle computer reading inaccurate.

Thanks,
-Joe

 

For some reason the standard factory calibration for many vehicles is 2 or 3 mph fast. Dakota Digital and Hypertech both make calibration units. You would need to search around here to find out hiw, and if, they work on our trucks.

 

I looked into the Hypertech device, looks like it might do what I am looking for. It is an inline device so it appears to be making the adjustment after the computer. The only issue I can see with this device is it appears to also adjusts the ODO which would be unfavorable.

I'll just live with it unless someone can tell me there is a calibration for the speedometer. I was hoping to run some calibration where the needle would point to "0", "15", "30", etc... and if the needle was off, there would be a software adjustment to make the needle move more linear. I'm sure there is something but it's likely a factory tool.

 

They do that so you can't sue them when you get a ticket.

Put on 265/75/16 (or similar) and your speedo will be correct - Bonus: your odo will claim you're travelling slightly fewer miles than you actually drive (preserve the already great resale value)