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 miles per hour isn’t as hard as you might think with a handy 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.
Speedometers: Calibration And Buying Guide
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.
The first and simplest way to find out if someone’s speedometer reading is wrong or if it isn’t properly calibrated will be when they’re thrown in front of a judge being charged with reckless driving. It’s not the most sophisticated calibration test, but it works. Unfortunately, it works too late. If reckless driving and speeding cases aren’t your thing, and an attorney at law is out of your budget, try one of these more sensible speedometer accuracy methods instead.
Accuracy of Your Speedometer
It’s challenging for your vehicle’s 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. Speed is determined by the number of driveshaft revolutions per mile. 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. An inaccurate speedometer could be the reason you received a speeding ticket…maybe?
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, but as long as you hit the button as close to the line as possible, you’ll have a rough idea of the amount that the speedometer is off by.
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 gear 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 speedometer calibrations 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 auto repair 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. While not as straightforward as an oil change, getting your speedometer calibrated properly doesn’t have to be complicated.
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 real-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 that measures magnetic field fluctuations..
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.