I have a '85 dodge d150 with a speedometer in need of calibration. Can anyone point me in the right direction to get this done?

 

the only way this can be done is by changing the speedo gear in the transmission. the hard part is figuring out what gear you need.

 

Agree with CW:
Here is a chart to help figure out which gear you need:
http://www.tciauto.com/tc/speedometer-gear-calculator/

 

My questions are, how do you know it is off? And WHY is it off. I guess the answer is related but you know what I mean!

I am guessing bigger tyres and the speedo is reading lower than actual MPH.

 

First, thanks to all for your responses. Each provided me with new information or something I hadn't thought about.
I really don't know if the calibration is off after answering SniperX questions. My daughter was following me on interstate. I was doing a steady 70mph by my speedo. Her car showed we were doing 80mph. My tires are standard for the the truck. The engine (318) didn't seem to be straining to maintain the speed as I would expect if I was driving 80mph. Maybe my daughters speedo is off.
Is there a test or better way to determine the speedo accuracy?

 

If you have a GPS it will read speed very accurately, if you don't have one you can usually borrow one. When I got my truck the speed sensor was broken and I had to use my GPS for a speedo and trip meter for three months before I both figured out why my speedo wasn't working and to figure out my mileage, and when I needed to fill up since the gas gauge was also wiggy.