ERB205 Ratio Calculator: Find the Correct DIP Switch Setting
Correcting a speedometer or tachometer signal is now much easier.
Enter the current reading and the desired reading, and the ERB205
calculator displays the nearest ratio, binary code and all eight DIP switch positions.
Click the image above to open the live ERB205 Ratio & DIP Switch Calculator.
Changing tyre size, differential ratio, transmission, engine, ECU or dashboard
can cause an electronic speedometer or tachometer to display the wrong reading.
The Bolec ERB205 Electronic Ratio Box
corrects a compatible frequency signal by applying a fixed adjustable ratio before
the signal reaches the gauge, ECU or control system.
The original ERB205 ratio chart remains useful, but selecting the correct eight-position
DIP switch setting manually can take time. The new online calculator performs the ratio
calculation and switch conversion automatically.
Calculate your ERB205 setting online
Enter the current or indicated reading and the desired or verified reading.
The tool shows the nearest available ERB205 ratio step, switch number,
eight-digit binary setting and the ON/OFF position of every DIP switch.
What can the ERB205 correct?
The ERB205 is designed for compatible variable-frequency signals in automotive,
marine and industrial applications. Typical uses include:
- Speedometer correction after changing tyre diameter
- Correction after differential, gearbox or drivetrain ratio changes
- Tachometer correction after an engine, ignition, alternator or ECU conversion
- Matching compatible factory sensors to aftermarket gauges or digital dashboards
- Applying a fixed ratio to suitable Hall Effect or magnetic-pickup signals
How the ERB205 ratio is calculated
Desired reading ÷ Current reading
The units are not important as long as both readings use the same measurement.
You can use km/h, mph, Hz or RPM.
Measure the current reading
Record the value currently shown by the speedometer, tachometer or receiving system.
Find the correct reading
Use a verified speed, frequency or RPM reading as the desired value.
Enter both values
The calculator selects the nearest ERB205 ratio and displays Switches 1–8.
Example: the speedometer reads 80 km/h, but the true speed is 70 km/h
Ratio = 70 ÷ 80 = 0.8750
The ERB205 setting is switch number 70, with binary code
01000110. This means Switches 2, 6 and 7 are ON.
The calculator keeps all eight DIP switches in one horizontal row so the
display matches the physical eight-position switch bank.
What the calculator displays
| Exact ratio | The mathematical result of desired reading ÷ current reading. |
|---|---|
| Nearest ERB205 ratio | The nearest available 0.0125 ratio step. |
| Switch number | The decimal value corresponding to the selected binary code. |
| Binary setting | Eight digits representing Switches 1–8 from left to right. |
| DIP switch positions | A clear ON/OFF display for all eight switches. |
| Predicted corrected reading | The calculated result using the nearest available ratio step. |
ERB205 signal and operating range
The ERB205 is specified for 12/24VDC negative-ground systems and accepts
compatible Hall Effect or magnetic-pickup inputs. Its standard frequency range
is approximately 5–500 Hz. The standard output is a square wave at approximately
50% duty cycle, with other output configurations available through the terminal
and internal-link options described in the instruction manual.
The ERB205 is a universal module, but it is not compatible with every sensor,
gauge or ECU. Confirm the input waveform, signal voltage, frequency range and
required output type before installation.
Why the calculator uses a warning below 0.025
The separate r8 switch chart includes setting number 1, equal to a ratio of 0.0125.
However, the ERB205 installation manual states a specified adjustment range of
0.025 to 3.1875. The online calculator therefore flags a requested ratio below
0.025 so the application can be confirmed with Bolec before use.
Setting the physical DIP switches
- Calculate the required ratio using the online calculator.
- Turn off power to the ERB205 before opening the module.
- View the switch bank with the ratio switch at the bottom of the PCB and the terminal block at the top.
- Set a binary 1 to ON / up and a binary 0 to OFF / down.
- Reassemble the module and test the corrected reading across the required operating range.
Switch 1 is the coarsest setting and Switch 8 is the finest setting.
Always verify the result after installation rather than relying only on the calculated value.
Ready to calculate your setting?
Open the ERB205 product page, enter your current and desired readings,
and copy the displayed eight-position DIP switch setting.
ERB205 documents and support
The full ratio chart and instruction manual remain available for installers
who need the complete terminal, wiring, PCB-link and setup information.
Download Ratio Chart
Download Instruction Manual
Watch ERB205 Video
Frequently asked questions
Can the calculator be used for both speedometer and tachometer correction?
Yes. The calculation uses desired reading ÷ current reading, so it can work
with compatible speed, frequency or RPM values when the same units are used for both inputs.
Does the calculator change the ERB205 automatically?
No. It displays the required setting. Power must be turned off and the
eight-position switch bank must then be set manually inside the ERB205.
What does a binary 1 or 0 mean?
A 1 means ON / up. A 0 means OFF / down. The eight digits correspond to
Switches 1–8 from left to right in the calculator.
Can the ERB205 correct every electronic speedometer?
No. The input signal, output signal, voltage level and frequency range
must be compatible. Contact Bolec with the application details when compatibility is uncertain.