Speedometer Calibration: Does Yours Need Checked?
Calibrate your speedometer to ensure you know how fast you’re going.
Published November 13, 2018
Speedometer calibration is essential for knowing how fast you’re traveling. Getting a speeding ticket has a way of ruining your entire day, so you want to pay attention to this, especially if you’ve had bigger tires installed. Thankfully, calculating your true MPH isn’t as hard as you might think.
The speedometer reads how many revolutions your tires make, not how fast they spin. When you upgrade to a bigger tire, this changes the circumference and makes the distance around longer. With each revolution that passes, you’ve traveled further than you did before. This makes your speed higher than what your speedometer is telling you. If you’ve gone down in size, then you are going slower than with your stock tire.
We’ll offer you an easy way to check and provide solutions for your speedometer calibration if you find the numbers are off.
Accuracy of Your Speedometer
It’s challenging for a speedometer to be entirely accurate. That’s why the majority of automakers design their speedometers to hit a certain tolerance range. Often this isn’t more than 1-5% slower or faster.
As long as you keep your car at factory specs, your speedometer should continue to fall within this range. Once the vehicle has been modified, however, the speedometer probably requires a recalibration.
The most common aspect that affects your speedometer calibration is the size of your tires. Large tires cover more ground during a revolution than a smaller tire. Consider this example:
If your factory-installed tires are 22.87-inches in diameter, the circumference is 71.81 inches. If you decide to replace those tires with 24.57-inch diameter tire, that circumference number changes to 77.13-inches. This means it travels over five inches further with every revolution. This is going to dramatically affect your speedometer, which now measures that you’re moving slower than you actually are.
How Off is Your Speedometer?
There’s a quick and easy way to figure the accuracy of your speedometer simply using a highway and a stopwatch. If you want to know how to check speedometer calibration, we’ve got the steps you need.
Find a stretch of highway that contains mile markers. Maintain a constant speed and have your passenger use the stopwatch to time how long it takes you to go one mile.
Then, you’ll divide 3,600 by the time it took you in seconds to complete that mile. This will give you an actual speed reading. So, if you set your cruise control to 60 mph and achieve the mile in 50 seconds, you’re real speed is 72 mph. That’s enough to get you pulled over.
This also tells you that your speedometer is reading 20% lower than it should. Of course, this isn’t entirely accurate. You must rely on the stopwatch to start and stop at the perfect moment and your speed to maintain precisely constant through the entire mile. There’s clearly a margin of error.
Mechanical Speedometer Calibration
With a mechanical speedometer, the gauge utilizes a flexible cable that runs from your transmission. The transmission output drives this cable with nylon gears. Your drive gear is the worm gear found inside the transmission. It runs off the output shaft. This turns the vehicle’s driven gear which is a pinion design that’s located on the end of your speedometer cable.
All you need to do is change the ratio found between your drive and the driven gear. Choosing a driven gear with fewer teeth speeds up the reading. On the other hand, having one with additional teeth will make the reading move slower. Changing the drive gear causes the opposite to occur: fewer teeth slows it down while more teeth speeds it up. Most people opt to change the driven gear because it’s at the end of the cable and easier to get to than the drive gear found in the transmission housing.
If you want to use our earlier example where the speedometer was off by 12 mph, you would need a driven gear that featured fewer teeth. The following formula works with many Chrysler, Ford, and GM vehicles to help you figure the number of drive gear teeth you want to use.
Take the number of drive gear teeth, multiply that by axle ratio and then multiply that again by tire revolutions per mile. Once you have that number, you’ll divide it by 1,001. This will give you the teeth you need on your driven gear to provide an accurate reading.
In reality, you don’t really need to figure this out for yourself. Most shops that sell speedometers will calculate the number for you. Just tell them the error, the number of teeth on your driven gear now, your rear tire diameter, transmission type and rear-end ratio.
Because this hasn’t been complicated enough for you yet, let’s take a minute to measure tires as well. You’ll measure your diameter from the ground until you reach the center of the axle. Make sure your tire is fully inflated and then multiply that number by two. This number provides a rear-world number because your tire is flattened where it rolls on your pavement.
If you want a more accurate reading, measure the rolling circumference instead. This is the distance a tire travels through one full rotation. Make a mark at the bottom center of your sidewall and on the pavement in the same spot. Then, roll the vehicle forward until the chalk hits the bottom again. Divide this distance by Pi (3.14159) to get the real diameter needed for our above equation.
There’s another way to handle calibrating your mechanical speedometer. The ratio adapter is just a small gearbox that alters the rotation speed of your mechanical speedometer’s rotation speed. You can install it anywhere in the system; at the speedometer or along the cable, but you’ll usually find them at the transmission.
Just ask the company you purchase your ratio adapter from and they’ll be able to help you find the properly geared adapter. They’ll need to know the same information as earlier to get you the right adapter.
Electronic Speedometer Calibration
Electronic speedometer calibration needs to be done slightly different because it doesn’t run on gears or cables. With that said, tire size changes and adjustments to the rear-end ratio still affect your speedometer calibration. To alter your calibration, you’ll need to swap the electronic ratio adapters. These control the number of electronic pulses instead of cable rotations through a mile.
What’s interesting is that adapters are on the market which allows you to use mechanical speedometers in conjunction with an electronic transmission and the other way around. While the mechanics of an electronic speedometer is similar to the mechanical counterpart, it runs through the variable speed sensor located at the transmission.
Electronic speedometers are more technical in nature, but much easier to calibrate because of the lack of mechanical parts involved.
All of the major companies offer programmable models. Some even come with push-button calibration which makes things even easier. An electronic speedometer can be calibrated many times over, just like you would reset your clock.
Once you’re done, you’ll want to recheck the speed to ensure that it’s accurate.
Best Speedometer Calibration Tool
There are plenty of speedometer calibration tools on the market, but you’ll want to be aware that most of them are specific to your model vehicle. We’ll list examples of some of the top choices, but it’s up to you to determine what will work for your make and model.
1. Dakota Digital SGI-5E Universal Speedometer Signal Interfaces
If you need a speedometer calibration for larger tires, this should do the trick. This system is set up to handle most all signal conversion or calibration issues once you’ve made changes to your tires, drivetrain or rear end gears.
It’s capable of recalibration anywhere from 2,000 ppm to 250,000 ppm so there’s plenty of versatility. The improved SGI-5E offers more flexibility than previous versions because it now allows up to 16-time multiplication. It also takes an 8,000 ppm signal and converts it into a 128,000 ppm output.
This system comes with a regulated 5V sensor output plus screw-in terminals. In addition, the push-button switches and LED display makes it simple and safe to perform your speedometer calibration. Make sure your speedometer is precise with an accurate tool like this one.
2. Ford Performance Parts Speedometer Recalibration Devices
If you want to change some specs to your Ford car, you’ll want the help of a high-quality recalibration device. Not only can messing with your gear ratios and tire sizes affect your speedometer, but it also messes with your engine control module, ABS, and other computer-controlled components.
With the Ford Performance speedometer calibration tool, you know that your speedometer, ABS, odometer, cruise control, and other calibration concerns will all be resolved. It’s easy enough for anyone to use. In fact, one user said it only took a half an hour for recalibration.
The best part is that there’s no reprogramming needed to use the tool. It scales a frequency based on the vehicle speed signals. This maintains an accurate speed indication to the speedometer and the PCM without any reprogramming needed. Of course, if you aren’t dealing with a compatible Ford vehicle, this tool won’t be ideal for you.
3. Superchips 3571 Flashcal F5 Tuner
If you want an advanced, easy to use tool for recalibrating your Wrangler, this is a great option. It helps you set things straight after upgrading your tires, axles, or gears. This quick solution ensures that your Jeep is properly dialed in. What’s unique about this design is that you have access to Flashpaq online. This update adds award-winning tuning ability to your device.
It’s probably one of the most advanced around while remaining simple to utilize. The advanced features also make it ideal for professional use as well. Just keep in mind that it’s only designed for select Jeep vehicles.
4. Hypertech 32501 Max Energy Power Programmer
This Hypertech speedometer calibration is one of the best on the market and well worth investing in. Of course, the high price reflects how great this tool truly is, but not everyone has this in their budget. The great thing about this programmer is that it works with gas or diesel engines alike. It also features an adjustable top speed limiter and engine rev limiter for use in some applications.
You can use this to correct speedometers and odometers in conjunction with tires up to 54 inches with this calibrator. Use it with your non-stock rear gear ratios as well to ensure accurate readings. If you want to avoid taking your car to the shop for calibration, this is the ideal solution. With the advanced features, you can make any adjustment you need right from your home garage.
It also reads and clears your diagnostic trouble codes making it completely versatile. Hypertech included a guide that’s simple to understand so even a beginner could figure this unit out. With a little reading, you’ll be ready to reprogram the car’s computer and have it reading accurately once again.
It also works on multiple vehicles, so it’s a great investment if you have more than one car you want to recalibrate.
5. Accu Pro Electronic Calibration Tool PC52001-1
When you want a speedometer calibration tool that works easily and effectively, you want this model from Accu Pro. With just a few simple touches of the buttons, you have exactly what you need to receive accurate readings after your modifications are complete.
This is a high-quality model with a lower price tag, so it’s more affordable than some other options. It also works on a variety of cars, so it’s perfect if you have more than one you need to calibrate. Though it is easy to use and a versatile model, it isn’t as simple to update as some other choices. If you want to save money though, this is a trade-off you’ll have to make.
Try out this plug and play device after you add bigger tires or change the gear ratios. You’ll find that this speedometer calibration tool helps you to set shift firmness, clears DTCs, and is straight-forward. You won’t need a wiring harness with this design and you can easily return your vehicle to the stock setting with the push of a button.
After you’ve upgraded your tires on your vehicle, there are other aspects you need to consider, such as your speedometer calibration. Don’t end up on the side of the road with a speeding ticket because you didn’t purchase a simple tool to fix your issues. With a wide variety of products on the market, at a range of prices, there’s no reason to go any longer without a calibrated speedometer.
If you have already performed speedometer calibration on your car, we would love to hear from you. What was the hardest part of the process and how would you advise others reading this article?
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