Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSRENAULT198518I/SPORTWAGON L4-2165CC 2.2LREPAIR AND DIAGNOSISTECHNICAL SERVICE BULLETINSALL TECHNICAL SERVICE BULLETINSA/T - MATH FORMULAS PART ISPEEDOMETER RATIOS
1985 Renault 18I/Sportwagon L4-2165cc 2.2L
Speedometer Ratios
1985 Renault 18I/Sportwagon L4-2165cc 2.2LSECTION Speedometer Ratios
Finally, let's look at speedometer ratios.
Suppose we put an exchange transmission in a car, and now the speedometer is off, because the speedometer drive gear has a different tooth count. What do we have to do to the driven gear to correct it?
Let's say the old drive gear had 7 teeth and the old driven gear had 21 teeth. The exchange unit had 8 teeth on the drive gear.
Old Drive Gear divided by the New Drive Gear = Ratio
7 teeth divided by 8 teeth = 0.875
Old Driven Gear divided by Ratio = New Driven Gear
21 teeth divided by 0.875 = 24 teeth
A 24 tooth driven gear will correct the speedometer error.
Let's do one more speedometer change. This time the old drive is 9, and the new drive is 10. The old driven gear is still 21.
Old Drive Gear divided by New Drive Gear = Ratio
9 tooth divided by 10 tooth = 0.9
Old Driven Gear divided by Ratio = New Driven Gear
21 tooth divided by 0.9 = 23.33 teeth
We can't get a 23.3 tooth count so we round it off to 23 teeth. Now the speedometer will be close, but not exact, because we had to round off the number.
Suppose we put an exchange transmission in a car, and now the speedometer is off, because the speedometer drive gear has a different tooth count. What do we have to do to the driven gear to correct it?
Let's say the old drive gear had 7 teeth and the old driven gear had 21 teeth. The exchange unit had 8 teeth on the drive gear.
Old Drive Gear divided by the New Drive Gear = Ratio
7 teeth divided by 8 teeth = 0.875
Old Driven Gear divided by Ratio = New Driven Gear
21 teeth divided by 0.875 = 24 teeth
A 24 tooth driven gear will correct the speedometer error.
Let's do one more speedometer change. This time the old drive is 9, and the new drive is 10. The old driven gear is still 21.
Old Drive Gear divided by New Drive Gear = Ratio
9 tooth divided by 10 tooth = 0.9
Old Driven Gear divided by Ratio = New Driven Gear
21 tooth divided by 0.9 = 23.33 teeth
We can't get a 23.3 tooth count so we round it off to 23 teeth. Now the speedometer will be close, but not exact, because we had to round off the number.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.