The tach reads 1650-1700 rpm @ 70 mph. From my calculations, it should read around 2300 rpm.
I don't have the capabilities to re-adjust the PCM. I'll check with a buddy that may be able to do this.
Thanks for the info.
sorry, what I meant to ask was if the error was consistent (i.e. a consistent percentage of what it should be across the board), or is it only off at specific times (like at idle, or above 2k, etc)....
a consistent error is what you sound like you're getting, something around ~25% off. so if you're idling at 600, it would be showing like 450. and when you should be at 3000, it'll show around 2250...
that would be likely due to the different crank reluctor and the signal the cluster is expecting. if you have a way to tune the computer yourself (like with EFILive or something), I can show you how to correct it, but if you're not tuned, or just using a mail order tune, then I'd try swapping the cluster stepper motors first, its highly likely that they're wound different or incorporate different internal components (resistors/capacitors/etc) to compensate for the different signals that are received. that would be the easiest and cheapest way to go about it.
if it were an intermittent error, like just at idle or something, then I'd say it was probably a gauge problem, like a mechanical or electrical failure/malfunction. but I don't think thats the issue you're seeing.