A speedometer is an essential tool for safe driving. Made standard for motor vehicles since the early 1900s, the speedometer has evolved over the years.
In contrast to its mechanical beginnings, today’s speedometers use more advanced technologies, sometimes even factoring in the use of GPS.
Regardless of the technology, all versions of the speedometer must accomplish the same task: accurately measure immediate speed. We need to know how fast we are going instantaneously. It is therefore imperative to have an accurate speedometer.
Recalibrating a more modern speedometer can be as simple as pressing a button and taking a drive, but if you have an older speedometer, or if you are trying to avoid a speeding ticket by showing inaccuracies in your speedometer, you will have to hire a mechanic.
The cost to recalibrate a speedometer is approximately $75 if you do not need parts and around $125 if you do need parts. Older vehicles with mechanical speedometers will need a replacement drive gear.
How does a speedometer work?
Because the size of the wheels is generally known, a speedometer measures how fast your wheels are going by measuring the rotations of the drive axle using electromagnetism.
Notably, if you put on different-sized wheels, your speedometer will no longer work properly and will need to be recalibrated. The speed measurement was initially done mechanically using a rotating cable and magnets.
In newer cars the principle is similar, but speed sensors replace the rotating cable method, and the measurement is often taken from the crankshaft or transmission output shaft. On the most modern cars, there are high-speed digital counters attached to the vehicle’s computer.
Should I bother recalibrating my speedometer?
There are several reasons why you should have a properly calibrated speedometer. First, having an incorrect speed measurement can be a safety hazard.
The fact is that speed limits are posted as an effort to maintain safe driving conditions, and not just to annoy drivers. In fact, roads are built with particular speeds in mind.
The speed recommendation is not random. Driving 45 mph on a curve with a speed limit of 30 mph could cause you to drive off the road.
Second, a correct speed measurement is relevant to several other systems. For example, if your car is measuring your speed incorrectly, it is also measuring your miles-per-gallon incorrectly.
Further, you will have inaccurate distance and time measurements when you try to reach a destination. If your GPS tells you that you will reach a destination in an hour if you travel 30 mph, then if you are traveling 35 mph, you will not arrive at the estimated time.
Finally, driving at the correct speed will save you time and money. If you are speeding and get a ticket you will have to waste time in court and likely have to pay a fine.
In fact, a speeding ticket is often the reason vehicle owners choose to calibrate their speedometer. For example, pretend that you get a speeding ticket for driving 50 mph in a 30-mph speed zone.
In many jurisdictions, this would be considered reckless driving, a charge with various steep consequences. In this case, you could get your speedometer tested and recalibrated.
If you can show that your speedometer was reading 44 mph when you were driving 50 mph, you can get a reckless driving charge reduced to a much more manageable charge.
Can I test the accuracy of my speedometer?
There are multiple ways in which you can test the accuracy of your speedometer.
Easy and less accurate
There is actually a very simple way to test the accuracy of your speedometer. If you are on a highway driving at 60 mph, you are also driving at a rate of one mile per minute.
Have a stopwatch handy. Start the timer as you pass a mile marker and stop it as you pass the next mile marker. If your speedometer is accurate, the time will read 60 seconds. If it reads more or less, your speedometer is not accurate.
Some math, more accurate
The above method will give you a reasonable estimate, but it is better to do this over a longer stretch. There is an easy mathematical formula you can use to help you.
For example, let’s say your time yourself driving 10 miles at a rate of 60 mph as it reads on your speedometer. To determine your actual speed, you would divide how many minutes it took you to drive your chosen distance by 60.
Next, you would multiply the previous result by the number of miles you traveled. Written as a formula it looks like this: 60 / time in minutes x distance = true mph
In the above example, if you drove 10 miles at 60 mph and it took you 9 minutes and 45 seconds, the calculation would look like this: 60 / 9.75 x 10 = 61.54-mph
This method takes an average over multiple miles and is therefore likely to be more accurate.
Use a professional
The final, and easiest method of testing your vehicle’s speedometer is to take it to a mechanic. The mechanic will use a tool called a dynamometer to do the test.
The dynamometer is a very expensive tool that will do an exceptional analysis of the accuracy of your speedometer. The report produced by the dynamometer can be used in court as evidence.
In fact, if your main purpose for having your speedometer tested is to have a speeding ticket reduced, you will need to hire a mechanic.
In general, the mechanic will have to be certified as a technician in the field, and he or she will likely need to sign the report. You will probably also require the services of a notary public.
How is a speedometer recalibrated?
There are different ways to recalibrate your speedometer depending on the type of speedometer you have.
Mechanical
Recalibrating a mechanical speedometer is a bit technical and may not be a plausible DIY project. You will have to replace the gear in your transmission.
First, you will locate the drive gear on your transmission and count how many teeth it has. You will also have to determine the rpms of your tires and find out your axle’s ratio.
Using these numbers, you will calculate the necessary size of the new gear. You will then have to do the actual replacement.
Electronic
In comparison, recalibrating an electronic speedometer is simple. You press a button, do a test drive, and the speedometer calibrates itself.
How much will it cost to recalibrate my speedometer?
The cost to recalibrate your speedometer will depend on the purpose for which you are recalibrating, and your location.
If you are not trying to avoid a speeding penalty and you have an electric speedometer, recalibrating is free. You simply press a button and do a test drive.
On the other hand, if you want to avoid a speeding ticket, or if you have a mechanical speedometer, you will want to employ a mechanic. A mechanic will charge for parts and labor.
For mechanical speedometers, the cost to recalibrate your speedometer will be the cost of labor plus the cost of a new speedometer drive gear. New drive gear can cost anywhere between $10 and $50, and it should take the mechanic less than an hour to do the job.
Depending on your location, the labor can cost you from $60 to $200 per hour. Considering all of this, you can take $100 as a reasonable estimate of the cost to have a mechanical speedometer recalibrated by a mechanic.
If you are trying to have a speeding ticket reduced and you have an electric speedometer, you will not have to pay for parts. The cost will be straight labor because all the mechanic must do is put the car on the dynamometer, run a few calculations, and certify the results.
Again, expect to pay for an hour of labor at the relevant rate in your area.
Conclusion
A properly calibrated speedometer is an essential tool for safe driving. Driving at the proper speed keeps you safe. Further many other vehicle systems depend on speed measurement.
You can do a simple accuracy test by driving at a constant speed and timing it, but if you need court evidence you will need a test certified by a technician.
Finally, although recalibrating your speedometer is sometimes as simple as pushing a button and taking a drive, there are a few reasons why you may have to pay a mechanic for help.
At most you should not have to pay for more than an hour of labor and the cost of one small gear. In the end, to remain safe and to avoid legal trouble, it may be a good idea to do a simple accuracy-test every few months or so.
It is easy, free, and fast to do. If there is a problem you can deal with it, but in any event, at least you will have peace of mind.