Speedometer question?

[Craftsmen]

Can someone help me out? Is the speedometer head in my dash on my 1970 C10 adjustable for calibration? I put 275-60-15 tires on the rear of my truck and my speedometer reads 10 mph slow. I have the Saginaw 3 speed with 3:73 differential Gears. As far as I know everything is original factory except the tire size. I haven't taken the speedo gears out of the tranny to know how many teeth they have. One of our members suggested to just purchase a converter gearbox to correct it. I called to order one and they told me that the speedometer should not consistently stay the ten mph difference at any speed. He said that when you speed up it should be a greater difference. He said I could have an issue with the head, that's why I'm asking about calibration. I'm confused and don't know what the best way to tackle the problem or where to start? Thanks for any information or guidance. I don't want to waste money on gears if it's the head and visa versa.

Just a note, the previous owner also had bigger tires on the back so the speedometer hasn't read correctly since I've Owned the truck. I don't know what size tire came on the truck from factory.