Transfer Case Speedo Gear Question

[MtBraun]

Up until today the speedo has not worked in my 78 k15. I have scrounged a gauge panel/speedo from a 74 2wd, and a cable from a 84 K10. It works but is measuring about 15mph slow in comparison to a gps. I pulled the driven gear and it is 18 teeth. It appears the only option is an external gear reducer?

I don't understand why its off in the first place. The truck has 2.76 gears and stock height 28" tires.

I also have a JTR speed sensor installed as part of the TBI conversion.

 

[highdesertrange]

do you have the ratio adapter installed or are you trying to put the cable straight to the transfer case? highdesertranger

 

[MtBraun]

I tried plugging the cable directly to the speed sensor, which is plugged to the TC. There was no speedo adaptor on the truck when we got it, so I assumed nothing was needed.

 

[highdesertrange]

a 1978 is supposed to have an external gear ratio adapter. highdesertranger