I recently swapped the speedo in my 1980 280zx 2+2, with one (seemingly identical) from an 82 coupe and it is by no means accurate. Now of course my question is why. I thought the gauge itself was the same and the cog on the different cars would cause it to be accurate therefore changing the speedometer alone should be a simple and effective swap. Please help.

 

Is it just as wrong as the previous one?

I need to know what rear-end you have and whether or not the transmission has ever been replaced.

It's most likely the differential was replaced with one that had different gears or the tranny was replaced and it has the wrong cog for your tranny.

Disconnect your speedo cable, pull the cog and count the teeth.

 

I'll check all that stuff but the old one was accurate. The reason I changed it is because it actually fell apart. First the needle broke and then the part on the back that the cable attaches to fell off and when I took it out I discovered that the gears inside the speedo almost looked as if they'd begun to melt at some point. As far as I know, all of the drive train parts are original.