There was some old info out there from freshalloy and zilvia, but I've recently stumbled across another inexpensive way to change the signal to the speedo. So I'm compiling information and also adding to it.
First and foremost, we'll go the free route...assuming that you're not bad with math, this one is super easy to do. The only thing I don't like about it, is the fact that if you constantly change wheel/tire/transmission set ups then you can potentially burn out your speedo and then this whole thing was pointless.
Start off by pulling your gauge cluster and removing the speedo from the housing. Once it is removed, flip it over to where the PCB is facing you and get out your solder sucker and your soldering iron. You'll see these 10 points on the back.
Each of these points represents a % of resistance.
ADJ 9: -49.4%
ADJ 8: -24.7%
ADJ 7: -12.4%
ADJ 6: -6.2%
ADJ 5: -3.1%
ADJ 4: -1.5%
ADJ 3: -0.8%
ADJ 2: -0.4%
ADJ 1: -0.2%
ADJ 0: -0.1%
So you can do some math and figure out how much you're off by, then desolder and resolder the points to meet your needs.
Now for the people who don't feel like soldering a PCB, there is another option that is even easier. You can go to http://shop.12oclocklabs.com/index.php ... duct_id=59
and get this speedo correction device (the universal one). It costs around $90, but you can change your speedo on the fly and you don't have to run the risk of burning up your PCB. Using the FSM (EL-78), will bring you to the combination meter wiring diagram.
Now with the speedo DRD you have 4 wires. The ground is going to go to pin 22 (black), 12V switched is going to pin 21 (green), then you'll cut the wire from pin 29 (Yellow/green) and the blue wire from the DRD will go on the harness side of the wire. The white wire will connect to the other side of the pin 29 wire going to the gauge cluster. 12oClocklabs has a really neat calculator on their website that will walk you through the percentage you need to change it and also exactly how to change it. It works based off the same concept as above, but you don't need to mess with any PCB's and it's a bit cleaner. Plus once it's done, you can change things on the fly and don't have to get dirty! If you don't want to pull the cluster, this same principal can be applied to the S14 F1 plug. If you want to use the F1 plug instead, the signal wire is going to be pin 24 (yellow/green).
For a DOHC S13 cluster, we're going to look at these. Just from a quick look at the FSM, using a 92 240sx application, EL-53 states it should be pin 12 (needle meter).
EL-57 states it should be pin 6 (digi meter). Those would be the pins used if you were running the unit from 12oclocklabs.
I personally like it at the cluster. This way you can drive down the street and if you need to make an adjustment, it's right there and you don't have to reach over or move to the passenger seat. Hope someone can get some use out of this!
I'd like to thank Gerry and Knate for their contribution to the community and knowledge.