I know some people are interested in the digital speedometer swap for their non-interceptor Lumina. There is scarce information out there on how to do this, so here is info on how I did it.
The hardest part of this typically is just tracking down a digital speedometer. If you can find one then half your battle is completed. One of the difficulties in using this speedometer is the trip/odometer switch and trip reset switch. I will show you how I resolved that as well, since I was unable to get the interceptor switches.

If you would have done things differently, that's fine, if you are looking to do this for yourself, keep in mind, I'm not telling you how to do it, I'm just saying how I did it, your results may vary.

I bought the cluster not knowing the milage on the odometer, so I had to hope and pray it was behind my odometer. When I got it and connected power to it, I was very relieved to see that it was only 3,600 miles behind my analog one at the time. Around 182,300 on the digital, and about 186,000 on my car. I wanted them to match, so I advanced the digital odometer to match the analog one.

The first thing to do was to reverse engineer the separate wiring harness, since the digital speedometer is just a drop in unit to a slightly modified analog cluster.
I found this wiring diagram online, and it was moderately helpful, but the colors did not match my unit, and the wiring order was different too, so I had to figure it out still. Here is the diagram, use at your own risk:


Wire colors and locations were different for me



Ever seen a Lumina go 255mph? Me neither.


Pardon the mess, but the mad science needs to happen somehow…

It was relatively straightforward to get the odometer to roll forward. I connected +12V to the power, 0V to ground, and then pulsed a signal to the VSS wire.
When the speedometer has power, VSS is at positive voltage and needs to be shorted to ground to indicate motion. there are 4,000 pulses per mile, so it only took a million or so to advance the odometer.
I just connected the VSS to the Source on a BS441 transistor. The Drain went to the odometer ground, and the Gate went through a 1K resistor to a cheapo Chinese signal generator which you can buy from the usual scumbags. I set the generator to square wave, 10% duty cycle, 1.5Volts, and then ramped up the speed. There are rumors the speedometer can handle 999mph, but I was not able to do that. The speedometer tops out at 255mph (just an 8-bit location in the computer). Ramping up the speed more would cause an overflow, so 256mph showed up as 1mph, etc. However, the odometer stopped advancing for me, so I recommend sticking to the 255mph limit. This took about 14 hours or so to advance the odometer to match. Why go through the trouble of this? Meh

Next was making a connector for the digital module. Since there are 8 wires, I got some nice fancy 8-pin Deutsch connectors from Aliexpress. Definitely recommend the barrel pins, rather than the wing type, but I already have a crimper for the wing connectors, and I'm cheap, so I didn't do that.



Do they look janky? Yes, but making a Deutsch connector for a 26 year old car is already extra credit, so stop bullying me.


Almost like I might have somewhat of a thought of an idea what I'm doing...

 

Next was the install. I'm assuming most of us have has the dashboard taken apart a few times, so no need to go into details there. (Consult your local Haynes or Chilton for help).

Install went pretty smoothly. The old connector just plugs right into the new cluster, but you need to manually wire in all of the other wires to get the speedometer to work.
Didn't take any pictures of this part of the built, but 12V into the digital module can tap off the 12V from the existing connector, same with ground and VSS.
The "Lights on input" connects to the brown wire on your headlight switch, and then "VF Dim" connects into the dimmer output wire, though I was disappointed to see it is not a variable dim on the screen, just bright, and slightly less bright, so both of these could connect to the brown wire on the headlight switch.
The English/Metric wire gets connected to ground for Metric, so I left a pigtail wire just floating, if someone in the future wants metric, they can dig the wire out from behind the cluster. (Hey, at least I gave future guy a pigtail wire from the connector, eh?)
Trip/Odometer select and trip reset were the fun ones. The trip/odo wire needs to be grounded to show the trip, and floats to show the odometer. I was looking at flip-flop circuits, as well as latching relays, which I'm sure would work in some cases, but ultimately I just tied this into my arduino microcontroller which I installed to control my side lights. The button just tells the microcontroller if I want trip or odometer, and then controls a transistor to sink the wire to ground if I want to see the trip meter.



Since I know you want to see the goods

This was bodged together to fix an issue with my side lights, and so I bodged in control of the trip/odometer as well.

Lastly, the trip reset wire gets sunk to ground to reset the trip meter, but only when in the trip display. On odometer this does nothing.


Here are photos of the odometer installed and working:


Old odometer for posterity

New one installed and working!

Proof the temp gauge and tach work

Proof my car goes faster than 7mph

 

Lastly, the buttons. I looked at many dozens of options from other cars, and finally decided the trip/trip reset buttons from a 2000 Oldsmobile Aurora were the ones for me. Same 52C interior color, as well as fitting the aesthetic of the Lumina. (Hey, moreso than from Toyota, or even the switches they hacked into the interceptor.)


Here is what the buttons look like:


The plastic was pretty gross on the housing, so I hit it with a 52C rattlecan:



Next I desoldered the pins and added these cute little wires:



Also replaced the bulbs with blue LEDs since all the switches in my car have blue LEDs:



Final assembly:

 

If you've taken the dash apart, you know there are not many places to be able to mount any switches, especially chonky boys like the buttons from the Aurora, so they had to go in the wallet/phone slot like in the interceptor. Again, since I couldn't find a panel or those switches, I had to make my own panel. Cutting the plastic was the hard part, mounting it was not too difficult. Since the back of the wallet/phone slot pops out and has clips, I just attached the new panel I made to that so it can clip in and be secure, yet also be removed.


I just used a piece of clear acrylic. I cut it in half, glued the two pieces together with hot glue, and then cut it down more with a table saw and jigsaw, and shaped it with a dremmel:



I popped the two pieces apart to paint the faceplate, drill holes in the back piece for the screws, and then glued the two pieces back together again:



Then I used some long bolts and all-thread to mount the front plate to the part that clips into the back of the wallet/phone slot:




I know it looks like poop, but it gets the job done:





In it's final home. Not bad not great 3/5 would recommend to a friend



Finally, some night shots to see the beautiful blue:

 

Last post with final details.

Correction: The transistor used to advance the odometer is a BD441

Here is the wiring for the digital module: (looking at the back of the instrument cluster, from the top left and going clockwise to the bottom left:
1: English/Metric (E: float, M: ground) [Light blue]
2: Vacuum Fluorescent Dim (Connect to +12V to dim) [Dark grey, blue stripe]
3: Vehicle Speed Sensor (+12V, 4000 pulses per mile) [Dark Green]
4: Ground [Black]
5: Trip Meter Reset (Ground for 0.5 seconds on trip display) [Light green]
6: Odometer/Trip Selector (O: float, T: ground) [Light grey]
7: Lights On (Connect to +12V to enable VF Dim) [Dark grey]
8: Power [Pink]





Final thoughts:
Not a mod to be done by the faint of heart. It's quite nice to have the speed digitally as it is faster to read. The brightness of the screen can get swamped out in early morning or evening when the sun is low and make it a bit hard to read. In very dark areas, even with the screen dimmed, it can be a bit bright, so if you live out in the country and are bothered by bright screens, don't do this mod. If you do this mod you will need to figure out your own way to control the odometer/trip select since programming a microcontroller will be out of the question for most people, just look for a latching On-Off rocker switch to control the odometer/trip, and a momentary (On)-Off switch to reset the trip meter.

Would I recommend this mod? Definitely. Assuming you have the tools and are comfortable with the necessary tasks to complete it.

I took apart the instrument cluster, mostly to reverse engineer the wiring, but also to clean out the spiderwebs and dust under the cover. There was a foam gasket between the glass of the VF display, and the instrument cluster front panel. This is to prevent light from the illumination lights from spilling into the display area and swamping out the digital screen. This foam turned to goop, and needed to be cleaned out and replaced to ensure light doesn't spill into the screen area when the parking lights are on. You will either need to not open the cluster at all, or if you do, then you will need to clean out this goop and replace it since it will no longer seal properly.