Swapped speedometer head, now different indicated speed

[Fbks_Cruiser]

I’ve been dealing with a chirping noise from the speedometer. I replaced the speedo cable when I got the truck years ago.. The noise just showed up intermittently last summer. I pulled the inner cable, cleaned and lubed it but still chirped on occasion. The last month or two it got more frequent and loud.

I also needed to replace the instrument printed circuit so I figured now is the time to try to fix the noisy speedometer since I was going to have to pull the cluster. I have a instrument cluster I pulled from a 1969 K20. So, I pulled the speedometer from it, rolled the odometer to match the mileage on mine, painted the needle and put it in my cluster. New printed circuit and bezel. Looks great! I also again pulled the inner cable, lubed it and verified no sharp bends, kinks or pinch points.

Took it out for a test drive today everything seemed to work great, nice and quiet with a steady needle. But, my ear and butt told me I was going faster than the speedometer said. I went back home and grabbed the GPS. The original speedometer read about 10% too fast. (60 mph indicated was more like 54-55 mph on the GPS). The new one reads about 15-20% too slow! ( 50 mph indicated was about 58 on the GPS). This difference agreed with what the “this is your speed” radar sign on the roadside said.

Is there really that much difference between factory speedometer accuracy?

BTW I’m running the NP205 transfer case and 235/85R 16 tires.

(The angle of the photo makes it look like the hash marks don’t line up with the numbers, but they do line up.)