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  • Bolec ERB205 Electronic Ratio Box 12/24VDC – Speedo corrector module
The Electronic Ratio Box is designed to give a permanent fixed ratio signal output from a continuously varying input signal.

Bolec ERB205 Electronic Ratio Box 12/24VDC – Speedo corrector module

$220.00

Available to order Bulk order pricing Delivery information Need help? 0480 312 400

ERB205 Electronic Ratio Box — 12/24VDC Speedometer & Tachometer Corrector

The ERB205 applies a fixed adjustable ratio to a compatible Hall Effect or
magnetic-pickup frequency signal. It is used to correct speedometer,
tachometer and instrumentation readings after tyre, differential, gearbox,
engine, ECU or dashboard changes.

  • 12/24 VDC negative-ground operation
  • Hall Effect and magnetic-pickup input capability
  • Specified ratio adjustment: 0.025 to 3.1875
  • 0.0125 ratio steps / approximately 1.25% increments
  • Standard frequency range: approximately 5–500 Hz
  • Square-wave, active, pull-down, open-collector and AC output options
  • Built-in ratio calculator displays all eight DIP switch positions

Need the correct ERB205 DIP switch setting?

Enter the current reading and the desired reading. The built-in calculator shows the nearest ratio, binary code and all eight DIP switch positions.

Open ERB205 Ratio & DIP Switch Calculator ↓

Product documents:
The ratio chart and instruction manual are available as direct PDF downloads.

View ERB205 Ratio Chart

View ERB205 Instruction Manual

Product resources
Download datasheet & instruction manualTechnical support
BOLEC Product catalogue / Signal correction module
Bolec ERB205 Electronic Ratio Box

Speedometer and tachometer signal correction

ERB205

Electronic Ratio Box — 12/24VDC

  • Hall Effect or magnetic pickup input
  • 0.025–3.1875 specified ratio range
  • Approx. 5–500 Hz standard range
  • 12/24VDC negative ground
  • 68 × 30 × 73 mm

bolec.com.au | sales@bolec.com.au | 0480 312 400

ERB205 technical overview

Correct speedometer, tachometer and frequency signals without replacing the gauge

The ERB205 converts a continuously varying frequency input into a corrected output at a fixed adjustable ratio. It is intended for compatible automotive, marine and industrial speed, RPM and instrumentation signals.

Use it after tyre-size, differential-ratio, gearbox, engine, ECU or instrument changes where the receiving gauge or controller needs a corrected frequency signal.
  • Speedometer correction
  • Tachometer correction
  • 12V and 24V systems
  • 1.25% ratio steps
  • Multiple output configurations
A4 catalogue layout — print or choose “Save as PDF”.
Product overview

Fixed-ratio correction for a variable frequency signal

The ERB205 accepts a compatible Hall Effect or magnetic-pickup signal and produces a corrected output based on the selected ratio. The standard output is a square wave at approximately 50% duty cycle.

Correct instrument readings

Adjust speedometer or tachometer readings after changes to tyres, gearing, drivetrain, engine, ECU or instrumentation.

Flexible signal interfacing

Supports compatible Hall Effect and magnetic pickup inputs with active, pull-down, open-collector and AC output options.

Simple ratio adjustment

Calculate desired reading ÷ actual reading, then set the eight-position ratio switch using the binary setting.
Application examples

Typical ERB205 uses

Compatibility depends on the signal waveform, amplitude, frequency and receiving equipment.

Tyre-size correction

Correct speedometer error after fitting tyres with a different rolling diameter.

Differential or gearbox changes

Re-scale the vehicle-speed signal after a differential ratio, transmission or drivetrain change.

Tachometer correction

Adjust a compatible RPM signal after engine, ignition, alternator, ECU or instrument changes.

Custom dashboards and conversions

Interface suitable factory or aftermarket gauges, ECUs and digital dash systems.

Marine instrumentation

Correct compatible speed or tachometer frequency signals in 12V or 24V negative-ground systems.

Industrial frequency conversion

Apply a fixed ratio to a suitable variable-frequency signal within the module's operating limits.
Interactive setting tool

ERB205 ratio and DIP switch calculator

Enter the current reading and the desired reading. The calculator finds the nearest available 0.0125 ratio step and displays the eight switch positions.

Use the same units for both readings. For a speedometer, enter the indicated speed as the current reading and the verified true speed as the desired reading.

Exact ratio 0.8750
Nearest ERB205 ratio 0.8750
Switch number 70
Binary setting 01000110
Predicted corrected reading 70.000 km/h
Difference from desired 0.000 km/h
8-position DIP switch Switch 1 → Switch 8
Within the installation manual's stated adjustment range.

Switch 1 is the coarsest setting and Switch 8 is the finest. A binary 1 means ON / up and 0 means OFF / down, viewed with the ratio switch at the bottom of the PCB and the terminal block at the top.

Ratio setting formula

Calculate the correction ratio

Ratio = desired / correct reading ÷ current / actual reading Example: 70 ÷ 80 = 0.8750. The r8 chart gives switch number 70 and binary setting 01000110.
Connections and output choices

ERB205 terminal connections

Select the output terminal and internal link configuration required by the receiving speedometer, tachometer, ECU or control system.
Terminal 1Positive 12/24 VDC supply, normally through the ignition switch and a 3 A fuse
Terminal 2Negative / ground
Terminal 3Square-wave push-pull active output
Terminal 4Square-wave analogue pull-down / open-collector output, 0–10 V or 0–V+
Terminal 5Hall Effect or magnetic pickup input
Terminal 6AC 1 active output
Terminal 7AC 2 analogue output, standard frequency ×10
Terminal 8Spare / not connected

Input switch

The input switches select low-level or high-level input operation and whether the input pull-up resistor is connected. Use the instruction manual for the exact positions.

Output level link

The standard link position provides approximately 0–10 V p-p. Move the link for an output matching the supply line, or remove it for the stated open-collector-to-ground configuration.
View the wiring diagram and PCB layout Open page 2 of the ERB205 instruction manual for the connector and link-position diagrams.
Ratio setup

Calculate, select and test the ratio

The readings may use km/h, mph, Hz or RPM, provided the same units are used for both values.

1. Measure the current reading

Record the reading currently received from the signal source or displayed by the instrument.

2. Calculate the ratio

Divide the desired reading by the current reading. Example: 400 ÷ 250 = 1.6000.

3. Find the binary setting

Use the calculator above or the r8 ratio chart to identify the nearest switch number and binary code.

4. Turn power off

Remove power before opening the module and changing the ratio switch.

5. Set switches 1–8

Set 1 / ON / up or 0 / OFF / down. Switch 1 is the coarsest and Switch 8 is the finest.

6. Reassemble and verify

Test the corrected reading across the required operating range before returning the vehicle or machine to service.
Ratio-chart note:The separate r8 chart includes setting 1 = 0.0125, while the installation manual states a specified adjustment range of 0.025 to 3.1875. Verify any requirement below 0.025 with Bolec before use.
Product data

Specifications

The controlled product documents and markings on the supplied module take precedence.
Part numberERB205
Supply12/24 VDC negative ground
Typical current drawApproximately 30 mA
Input typesHall Effect or magnetic pickup / inductive signal
Input levelHall Effect approximately 0–10 V p-p; magnetic pickup approximately 2.5–50 V
Standard frequency rangeApproximately 5–500 Hz, regardless of ratio
Modified rangeApproximately 30 Hz–3 kHz; refer to the wiring document
Specified ratio adjustmentFOUT / FIN from 0.025 to 3.1875
Ratio step0.0125, equal to 1.25% of input frequency
Standard outputSquare wave, approximately 50% duty cycle, 0–10 V p-p
Alternative outputs0–V+ square wave, active output, open collector, AC output and AC output ×10
Dimensions68 × 30 × 73 mm deep
Mounting83 mm hole centres; approximately 35 mm mounted height
Installation clearanceAllow 110+ mm depth with connector and wiring
Product video

See the ERB205 application overview

Watch the existing Bolec ERB205 product video.
ERB205 electronic ratio box product video.

Product video

Watch online: youtube.com/watch?v=bJSqeWSs428
Frequently asked questions

ERB205 FAQ

Quick answers about ratio calculation, signal types and installation.
How do I calculate the ERB205 ratio?
Divide the desired or correct reading by the current or actual reading. Example: 70 ÷ 80 = 0.8750.
What do the binary digits mean?
A 1 means the corresponding DIP switch is ON / up. A 0 means OFF / down. Switch 1 is the coarsest setting and Switch 8 is the finest.
Can the ERB205 correct a speedometer after changing tyre size?
Yes, where the vehicle uses a compatible electronic speed signal. Enter the indicated speed and verified true speed into the calculator, then confirm the signal type and installation with the manual.
Can it correct a tachometer?
Yes, for compatible Hall Effect, magnetic pickup, alternator or other frequency-based tachometer signals within the stated limits.
Does it work on both 12V and 24V systems?
It is specified for 12/24 VDC negative-ground systems.
Is the ERB205 compatible with every vehicle or gauge?
No. Confirm the input waveform, signal level, frequency range, output type and receiving equipment before installation.
Documents and tools

Downloads and technical support

Use the ratio calculator for quick selection and the controlled documents for installation.
Download ratio switch chart Open the ERB205 r8 ratio and binary switch-setting table. Download instruction manual Installation, terminals, wiring diagram, setup and specifications. Application support Ask about signal compatibility, output selection or special ratio requirements.

The controlled Bolec document and markings on the supplied product take precedence if any difference is found.

Application guides

Related guides

Read practical application notes and product guides from Bolec Electronics.

Bolec Products · August 3, 2026

ERB205 Ratio Calculator: Find the Correct DIP Switch Setting for Speedometer and Tachometer Correction

New online setting tool ERB205 Ratio Calculator: Find the Correct DIP Switch Setting Correcting a speedometer or tachometer signal is now much easier. Enter… Read guide

View all Bolec guides

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