How do you convert from a mechanical speedometer/tachometer to an electronic one?

You should look for a qualified repair shop , unless you know how to test automotive electrical circuits . Battery drain or parasitic draw on the battery is not easy to find if not familiar with automotive electrical testing . Also having wiring diagrams an service repair info . Does the truck run an drive ok ? The speedo an tach not working could be a problem inside the instrument cluster or VSS - vehicle speed sensor for the speedometer .
Vehicle Speed Sensor (VSS)
On vehicles equipped with 4-wheel anti-lock brake systems (4WABS), the vehicle speed signal is generated by the rear anti-lock brake sensor and sent to the 4WABS module. The 4WABS module sends the vehicle speed signal via circuit 679 (GY/BK) to all systems that require a vehicle speed signal input. Vehicles equipped with rear anti-lock brakes (RABS) generate a vehicle speed signal from the rear axle speed sensor. The generic electronic module (GEM) receives this signal (VSS_GEM) for internal use and then distributes it to the appropriate other users (i.e., the powertrain control module [PCM], speed control module, and speedometer).
Speedometer
The electronic speedometer (17255) receives a speed signal from the 4WABS module (if equipped with 4WABS) or the generic electronic module (GEM)/central timer module (CTM) (if equipped with RABS).

Tachometer
Gasoline vehicles are equipped with a 6,000 rpm tachometer and diesel vehicles are equipped with a 4,500 rpm tachometer. The tachometer is hardwired to the PCM through circuit 648 (WH/PK).

https://www.youtube.com/watch?v=efKoWifdcag&t=844s

https://www.youtube.com/watch?v=6xFo_oHFF48&t=2056s

You might have to replace the speedometer cable

Older cars run their speedometers from cables. I think in your case it's measured electronically. Check the module on the gearbox. This is where it gets its signal from. There is a gear on it. The gear measures the speed mechanically. Then convert it to an electric pulse. Paired to your speedometer.

Tachometer / Speedometer Problems: - Ford Explorer and Ranger ...

www.explorerforum.com > ... > Under the Hood

Jul 31, 2006 - 13 posts - ‎11 authors
Tachometer / Speedometer Problems: Under the Hood. ... upon restat the speedo goesto (is driven to) zero, the tach goes to ... 88 89 93 95 96 Aerostars ... in cold weather it seems to stick. but once the truck's interior warms up, .... 1995 Ford Explorer 4.0Automatic My Tach Quit Working And So Did My ... 5R55S harsh shifting; bump engaging reverse;
17 posts
Sep 3, 2011
Not starting - speedometer / tachometer needles going ...
3 posts
Jun 29, 2011
Tachometer Problem
12 posts
Nov 7, 2009
Solved 5R55E - No First Gear Problem
19 posts
Dec 7, 2000
More results from www.explorerforum.com

1999 Ford Explorer , Erratic RPM Problem - YouTube

The speedometer is operated differently then the tachometer. Tachometer
The tachometer displays the engine speed in revolutions per minute (RPM). Voltage pulses are taken from the ignition control module (ICM) (Gasoline) or the generator (Diesel) and sent to the tachometer. Solid state circuits convert the pulse frequency in order to move the pointer of the tachometer.

Tachometer Inoperative (Gas)
Step
Action
Value(s)
Yes
No
1
Did you perform the OBD II System Check been performed?
--
Go to Step 2
Go to Powertrain On Board Diagnostic (OBD) System Check (5.0L/5.7L); Powertrain On Board Diagnostic (OBD) System Check (7.4L)
2
Remove the I/P Cluster.
Disconnect the Ignition Coil connector.
Connect a J 39200 DMM (continuity) between I/P Cluster connector cavity 6 and ground.
Is continuity present?
--
Go to Step 3
Go to Step 4
3
Locate and repair the short to ground in CKT 121 (WHT) between the Ignition Coil and the I/P cluster. Refer to Wiring Repairs in Wiring Systems.
Is the repair complete?
--
Refer to Instrument Cluster System Check
--
4
Connect a J 39200 DMM (continuity) from I/P Cluster cavity 6 to ignition coil connector cavity B.
Is continuity present?
--
Go to Step 5
Go to Step 6
5
Replace the I/P Cluster. Refer to Instrument Cluster Replacement .
Is the repair complete?
--
Refer to Instrument Cluster System Check
--
6
Locate and repair the open in CKT 121 (WHT) between the Ignition Coil and the I/P Cluster. Refer to Wiring Repairs in Wiring Systems.
Is the repair complete?
--
Refer to Instrument Cluster System Check

If it used the mechanical speed cable. Check the speedometer cable attachment at the speedometer in the instrument panel. Check the speedometer cable and speedometer sending unit assembly on the transmission.

Sounds like a bad speedometer sensor. Usually located on the transmission. The only way to fix the speedometer is to replace the sensor.
The Variable Speed Sensor (VSS) has failed. You will have to get it replaced. Both are driven by an electrical signal sent to the cars computer by the VSS, and converted to SPEED, and DISTANCE driven.

Both are in need of repair, nothing else will cause this kind of problem except the inner electronics of both instruments, they can be repaired by any authorized Hyundai electronics repair station, but it won't e cheap.

The speedometer is a mechanically driven gauge so you need to check the cable is pushed in at the back. The tachometers on several model Charades are known to act up. There has been varying success by resoldering dry joints on the cluster PCB and the gauge itself.