ERB205 – Electronic Ratio Box for Speedo & Tacho Calibration
Perfect for retro builds, 4WDs and classic cars with non-standard tyre sizes or gearbox swaps.
When your speedo doesn’t match your wheels
Anyone who’s swapped diff ratios, changed tyre sizes or converted a classic to a new transmission knows the feeling — your speedometer suddenly lies to you.
Bigger tyres mean your speedo reads low. Smaller ones, it reads high. That’s where the Bolec ERB205 Electronic Ratio Box comes in.
Designed and built in Australia, the ERB205 gives you full control to calibrate any electronic speedo or tacho signal after mechanical changes —
without pulling apart the dash or gearbox.
What it does
The ERB205 takes an input signal (from a Hall effect or magnetic pickup sensor) and outputs a corrected, fixed-ratio frequency.
That means you can fine-tune how your speedometer or tachometer interprets the signal — matching your new drivetrain setup precisely.
- Adjustable ratio from 0.025× to 3.18× (divide or multiply the input signal)
- 8 Way DIP switch for precise 1.25% increments
- Works with Hall effect or magnetic pickup signals
- Compatible with both 12V and 24V systems
- LED indicator for input signal activity
Why 4WD and retro car builders love it
Many 4WD owners upgrade to larger tyres or different transfer cases for off-road performance — but that change throws off speed readings.
Likewise, retro car enthusiasts fitting late-model gearboxes or EFI conversions often face mismatched tachometer signals.
The ERB205 bridges old-school hardware and modern electronics, letting you dial in the exact ratio needed for accurate readings again.
No reprogramming ECU, no aftermarket dash, no compromise.
“If your speedo says 90 when you’re actually doing 100, the ERB205 can fix it — permanently.”
Easy setup, precise results
You don’t need a laptop or software. Just remove the lid and set the DIP switches using the supplied chart.
Calculate your ratio — for example:
- Your speedo shows 80 km/h but GPS says 100 km/h → Ratio = 100 / 80 = 1.25
- Set DIP switches to the binary code for 1.25 (01011010) — done!
Fine-tuning can even be done by trial and error — increase or decrease one switch at a time until the display matches your GPS speed.
Where to install it
The ERB205 can be mounted under the dash or near the fuse panel. It connects easily:
- +12/24 V → Ignition supply (via 3A fuse)
- Ground → Good chassis earth
- Input → From Hall effect or magnetic pickup sensor
- Output → To speedo or tacho input
Built to last
Like all Bolec modules, the ERB205 is fully encapsulated and vibration-resistant, built for harsh environments —
from outback 4WD touring to marine engine bays.
Whether you’re restoring an old Patrol, tuning a Hilux conversion, or building a classic with new running gear,
this small box saves hours of guesswork and keeps your gauges honest.
Technical Summary
| Supply Voltage | 12 / 24 V DC (negative ground) |
| Input Type | Hall Effect or Magnetic Pickup |
| Adjustment Range | 0.025 – 3.1875× (divide/multiply) |
| Frequency Range | Approx. 5 – 500 Hz (optional up to 3 kHz) |
| Output Signal | 0–10 Vp-p or 0–V+ square wave (50% duty cycle) |
Need help setting the ratio?
We can help calculate the ideal DIP switch setting for your build. Just tell us your:
- Vehicle type & transmission
- Tyre size or diff ratio
- Current vs. desired speed reading
Our team can guide you to the correct ratio so your gauges show true readings again.
Bolec Electronics – Australian-made calibration modules for speed, RPM and sensor control since 1983.