That's a god one. Both of my v6 turned 4 cylinder tachs read too low until I adjusted them. Except for being at 5K, does the tach work normally? I mean, when you rev on it does the tach go up? When it come to simplicity, I would tend to lean towards a bad tach. I say this because only three wires run the tach, power ground and tach input. As for the tach, it has wound wires and integrated circuit boards. Do you have a cheapo aftermarket tach you can get a hold of? I would say using your wiring diagram, find the wire at the back of the cluster that is the tach input and hook it to your cheapo tach. If the cheapo tach works off the same wire that feeds the stock tach, then I would say the tach is bad or that needle is still not on right. Also check for proper voltage and ground to the gauge.
On another note, sr5 clusters appear on ebay every once in a while, at decent prices. I now own three. Why? Just for this reason, no only can I swap gauges to see if the gauge is bad, but if it is, I already have a spare. It came in handy when my temp gauge went bad recently. Trust me, you don't want to go to toyota to get a replacement gauge.
