Replacing your vehicle’s tires with bigger ones is not bad. It improves handling, cornering, decreases cornering distance, and increases acceleration. But it is essential to know that using bigger tires would alter your speedometer calibration, and you may need to recalibrate it to get an accurate reading of your vehicle’s speed.
A speedometer will read a speed slower than the vehicle’s actual speed when you replace your standard tires with larger ones.
You may need to calibrate your vehicle’s speedometer after replacing your tires with new ones, especially if the new tires are of different sizes.
There are two types of speedometers, mechanical speedometers and electronic speedometers. Each is calibrated using a different method. The following is a guide on how to calibrate the two types of speedometers.
To adjust your analog speedometer gauge for bigger tires, first, count the number of teeth on the drive gear. Then, calculate the tire’s number of revolutions per mile, then replace the old driven gear with the right one.
Calibrating a mechanical speedometer is a complex process. One may need the help of a technician since it involves replacing some parts.
You start by locating your transmission. Transmission can be pin-pointed by opening the hood or lifting your vehicle and searching for it from the bottom.
The speedometer cable is normally bolted on the transmission’s tail shaft. Unbolt the speedometer cable from the tail shaft.
Remove the gear housing cover by unbolting the screw holding it in place. Using an adjustable spanner would be quicker than any other spanner. You will see two gear. The speedometer cable is attached to the driven gear, while the drive gear is attached to the transmission.
Count the number of teeth on the drive gear and note down.
Determine the diameter of your wheel by measuring from the top to the center of the wheel. Then multiply this value by two to get the diameter. To get the tire’s revolutions per mile, divide 20,168 by the diameter of the tire.
Multiply the tire’s number of revolutions per mile by the value of the number of teeth on the drive gear. Then multiply the value you get with the axle’s ratio.
The axle’s ratio is normally found on the vehicle’s manual or inside the vehicle’s glovebox. Divide what you get with 1,001.
The value you get is the number of teeth of the driven gear needed so that your speedo gives accurate vehicle speeds. Provide your local dealership with the number of teeth of the driven gear, the make of your vehicle, model and model year of your car to ensure they deliver the right gear.
Replace the old gear with the right one and attach the speedometer cable at the right place. Return everything to its position.
Identify the speedometer calibration button and press-hold, crank your vehicle and then release the button. Push the button and drive the distance recommended by the manufacturer. After driving, press the button again, and the speedometer will automatically calibrate itself.
Electronic speedometers are easy to calibrate. Manufacturers always provide the distance of the test drive needed for speedometer calibration.
This information is normally available in the owner’s manual.
Improved Grip and Traction
Larger tires have a larger surface area in contact with the track.
This enhances grip and traction and improves the vehicles handling. Bigger and thinner tires offer more traction on snow.
Enhances Cornering
Bigger wheels with wide threads and stiffer walls offer more road contact enhancing cornering at high speeds. This is the main reason racing cars have more massive and wider wheels.
Improved Breaking
Big tires will offer more surface contact with the track. This enhances braking and reduces braking distance.
Better Appearance
Vehicles with bigger and wider tires look better than those with smaller tires.
Hard Steering
A larger tire has more contact with the surface making it very hard to steer.
A larger tire also means more weight to be turned by the steering system.
Increased Fuel Consumption
Installing bigger tires would increase the weight to be turned by the engine, leading to more fuel consumption. But this weight increase would not have any effect on vehicles with powerful engines.
Increased Tire Wear
Larger tires wear out faster due to lower sidewalls, wheel scraping, and stiff steering.
Inaccurate Speedometer Reading
Installing a larger wheel without calibrating your vehicle’s speedometer would result in inaccurate readings. Calibration may require the replacement of driven gear, which may be more expensive.
Safety System Warning
Various safety warnings would appear on the dash due to the wrong tire size. Since the wheel speed sensors work together with the engine control unit, traction control, and the ABS, you may get an ABS or traction control warning light.
Many people get confused about what the letters and numbers on the side of their tire wall stand for.
We will use a case study to explain what these letters and numbers stand for and how to determine the size of your wheels. Let’s assume your wheel is numbered P215/65R/15 95H.
Tire Type
The first letter “P” refers to the standard to which the tire is made. The tire is a P-metric tire, the standard used in the United States. This standard shows that this tire is designed for passenger vehicles. If the tire starts or ends with the letter “LT,” this means the tire is designed for light trucks. Always refer to your owner’s manual to know which type of tire is suitable for your vehicle.
32′ x 9.50″ BFGoodrich KO2 All-Terrain Tire on my 1993 Jeep YJ SaharaTire Width
Tire width is the distance between the two sidewalls. The first three digits represent the tire width in millimeters. In our case, the tire width is 215 millimeters.
Aspect Ratio
The Aspect ratio is the ratio of the height of the tire cross-section to its width.
The two-digit number after the backlash mark represents the tire’s aspect ratio. The larger the ratio, the bigger the aspect ratio.
Construction
They are various types of tire construction. It may be radial, basic, ply, etc. “R” stands for radial, which means the tire is layered radially.
Wheel Diameter
The two-digit number after the letter “R” is the diameter of the wheel in inches. In our example, the wheel is 15 inches in diameter.
Speed Ratings
The last letter on your tire label represents a certain speed on the speed chart. This shows the stopping power of the wheel.
Tires with high-speed ratings have better stopping power and less durable threads.
In this case, the last letter is H, meaning the speed rating is 210 km/h.
Tire Width
Tires with a broader profile offer more grip than those with a narrow profile.
Tire Profile
Tires with high profile offer a comfortable ride and provide more protection. Tires with a low profile are prone to the same. They give a rough ride but are better looking and enhance steering performance.
Manufacturer’s Warranty
Different manufacturers offer different warranty terms. There is often a list of damages that a warranty can cover. Some manufactures offer a very narrow list of damage that their guarantee can cover, while others cover more. Always check the terms of a warranty before buying.
Tread Pattern
There are three types of treads; symmetrical, directional, and asymmetrical. Symmetrical tires offer quiet performance but offer substandard grip on rainy weather. Asymmetrical tires perform well in rains as they disperse the water well but are more expensive.
Directional threads function best in rains but produce so much noise.
Installing bigger tires improves acceleration, handling and makes your vehicle look good, but it is essential to recalibrate your speedometer to get correct vehicle speed readings.
Thank you for reading this article. Please feel free to comment and share.
by Alexander Poirier
speedometer image by Larry from Fotolia.com
A vehicle's speedometer gives the driver an accurate estimation of the vehicle's speed. The speedometers on stock vehicles are calibrated for use with the vehicle's stock parts. Changing the stock parts, including the vehicle's tires, can throw off the speedometer calibration; larger tires cause the speedometer to read a speed slower than the vehicle's actual speed while smaller tires cause the speedometer to read a speed faster than the vehicle's actual speed.
You may recalibrate a speedometer in a few simple steps.
Open the vehicle's hood and locate the transmission. Unscrew the speedometer cable from the transmission's tail shaft and use a crescent wrench to unscrew the bolt holding the gear housing cover in place. Remove the plate to expose the two gears: the drive gear mounted inside of the transmission and the driven gear attached to the speedometer cable. Count the number of teeth on the drive gear and note the number.
Measure from the top of one of the vehicle's wheels to the center of the wheel. Multiply this number by two to determine the diameter of the tire. Divide 20,168 by the tire's diameter to determine the tire's revolutions per mile.
Multiply the vehicle's revolutions per mile by the amount of drive gear teeth determined in Step 1. Multiply this value by the vehicle's axle ratio. Look for the vehicle's axle ratio in the supporting documents that came with your vehicle.
It may also be found inside the vehicle's glove box. Divide the solution by 1,001. The resulting number is the number of driven gear teeth needed.
Contact the vehicle's manufacturer or a local speedometer service location and purchase the necessary driven gear for your vehicle. You must provide the make, model and year of your vehicle to ensure you receive the right gear.
Remove the old driven gear from the end of the speedometer cable by pulling it out of the clips that hold it in place. Clip the new driven gear into place on the speedometer cable. Replace the gear housing cover and secure it in place with the bolt you removed in Step 1. Secure the speedometer cable back into position.
Determine the distance of the test drive needed to recalibrate your speedometer. This information may be in your owner's manual or other supporting documentation, but it is also available from the vehicle's manufacturer.
Press and hold the calibration button on the speedometer, start the vehicle and release the button.
Press the button again and drive the distance prescribed by the speedometer manufacturer.
Press the button a third time after you have driven the designated distance. After pressing the button the third and final time, the speedometer will recalibrate itself to fit the new tire size.
Things You'll Need
Writer Bio
Alexander Poirier began writing professionally in 2005. He worked as the editor-in-chief of the literary magazine "Calliope," garnering the magazine two APEX Awards for excellence in publication. Poirer graduated from the University of the Pacific with a Bachelor of Arts in English.
Question: "How much will the speedometer lower the speed when installing larger tires" V1/V2 = (12.
7*R1 + 0.01*h2*P1)/(12.7*R2 + 0.01*h2*P2)
where V is the speed, R is the rim diameter, H is the width of the tire profile, P is the relative height of the tire). Tire sag is assumed to be the same.
As a result, if you change the nominal 185/70 R14 to the nominal 190 / 65R15 - with the same speedometer readings, the real speed and mileage will increase by three percent. But... Since, for safety reasons, the design assumes that the speed on the speedometer will always, even on the largest nominal wheels, be greater than the real one, then this is all uncritical...
P.S. If you don't want to count manually, here is one of the calculators: http://www.gdtm.ru/tirecalc.htm
Andrey 78 -- 2003-03-11 15:23:57
Eugene, your opinion here is probably the most authoritative, but is it true that the design of the car is (quote): "the speed on the speedometer will always be, even on the largest nominal wheels, more than the real one"
It seems to me that the speedometer is a device simply _obliged_ to show the exact value of the speed.
It is accurate, and not underestimated or overestimated, because safety depends on it in many cases.
And what's the point of distorting the speedometer readings for the sake of safety?
It may very well be that I am mistaken, but as proof of my words, could you provide a link to a source that would say that the speedometer readings do not correspond to the actual speed of the car? (After all, any non-compliance is a crime)
Board(77) -- 2003-03-11 18:54:10
___Pete's pipe is completely optional. With a special desire, you can make a speed corrector on any microcontroller (of course, provided that the speedometer is electronically driven, of which the majority are now). As for the accuracy of speedometers, they really have a positive error. A long time ago (when I didn’t know about the error) I was surprised that on my different cars, when I got caught by the traffic police for speeding, the measured speed was always lower than what my speedometer showed. Then I repeatedly listened to people's stories about how their serial Zhigul (without work on a real increase in engine power) "squeezed out" 170..180 km / h (well, well ...). Then, in the end, in the literature, I found information about the positive error of speedometers. I saw the information, perhaps even more than once, though I can’t give a link now. I think that having rummaged in the I-net, this information will not be difficult to find. nine0013 ___ The need for this positive error is easily explained. The speedometer (especially mechanical), like any other measuring device, has an error. It consists of the error in the manufacture of parts (geometric dimensions, magnetization of the inductor, etc.), assembly (it can be assembled in different ways), etc. In addition, temperature errors, etc. are added. Plus, during operation, the accuracy changes (vibrations, mechanical wear, demagnetization etc.). Here I gave some points regarding mechanical speedometers, but speedometers with an electronic drive and a pointer device have other errors that are basically similar to those given.
Plus, during operation, the tires decrease in diameter with all the consequences. So, it is NECESSARY that, taking into account all these possible errors of the speedometer, the car should have a speed NOT BIGGER than the one that the speedometer shows. By introducing a certain positive error into the speedometer, it is possible with a given probability to ensure the fulfillment of this requirement even taking into account the remaining (above) errors. nine0013 ___ P.S. I just typed the phrase "positive error of the speedometer" on Yandex, received 3 documents. Here is a quote from one of them: "The average error of the speedometer according to UNECE No. 39 can only be positive and not exceed the true speed by more than 10% + 6 km / h. In other words, when driving at a speed of 170 km / h. the speedometer has the right to show 170+17+6 = 193 km/h."
I would like to hope that the reference to the document "UNECE No. 39" will be authoritative enough :-)))
___ Today, for non-contact speed measurement, speedometers based on a high-precision acceleration sensor (accelerometer) are used, integrating its readings over time.
I don't know how much this thing costs, but I think it's quite a lot, because the acceleration sensor must be VERY accurate, because the error is also integrated and constantly accumulates. In this regard, a GPS-based speed meter should be much cheaper (and the achievable accuracy can be quite acceptable). nine0013 ___ However, in the end, I would like to see the results of the adjustment on the speedometer :-). And for this you need a corrector ... In principle, the thing is simple, but you need a program for it. For a particular case, i.e. to convert the input speed by X percent, it's easy. But for a wide application, you need a menu that will allow the user to enter his desired adjustment, and this is much more complicated ...
Source: toyota-rus.narod.ru
It is no coincidence that the speedometer is located in the most visible place on the dashboard of a car.
After all, this device shows how fast you are driving, and allows you to control compliance with the permissible speed limit, which directly affects road safety. Let's not forget about speeding tickets, which can be avoided if you periodically glance at the speedometer. In addition, on country roads with the help of this device, you can save fuel if you maintain the optimal speed at which fuel consumption is minimal. nine0007
The mechanical speedometer was invented over a hundred years ago and is still widely used in vehicles. The sensor here is usually a gear that meshes with a special gear on the secondary shaft of the gearbox. In front-wheel drive vehicles, the sensor can be located on the axis of the drive wheels, and in all-wheel drive vehicles, in the transfer case.
The speed indicator (6) on the instrument panel is a pointer instrument based on the principle of magnetic induction. nine0013 The transmission of rotation from the sensor (1) to the speed indicator (actual speedometer) is carried out using a flexible shaft (cable) (2) from several twisted steel threads with a tetrahedral tip at both ends. The cable rotates freely around its axis in a special plastic protective sheath.
The actuator consists of a permanent magnet (3), which is mounted on a drive cable and rotates with it, and an aluminum cylinder or disk (4), on the axis of which the speedometer needle is fixed. The metal screen protects the structure from the effects of external magnetic fields, which could distort the readings of the device. nine0013 The rotation of the magnet induces eddy currents in a non-magnetic material (aluminum). Interaction with the magnetic field of a rotating magnet causes the aluminum disk to rotate as well. However, the presence of a return spring (5) leads to the fact that the disk, and with it the pointer arrow, only rotates through a certain angle proportional to the speed of the vehicle.
At one time, some manufacturers tried to use tape and drum-type indicators in mechanical speedometers, but they turned out to be not very convenient, and they were eventually abandoned. nine0007
Despite the simplicity and reliability of mechanical speedometers with a flexible shaft as a drive, this design often gives a rather large error, and the cable itself is the most problematic element in it. Therefore, purely mechanical speedometers are gradually becoming a thing of the past, giving way to electromechanical and electronic devices.
The electromechanical speedometer also uses a flexible drive shaft, but the magnetic induction speed assembly in the instrument is arranged differently. Instead of an aluminum cylinder, an inductor is installed here, in which an electric current is generated under the influence of a changing magnetic field.
The higher the speed of rotation of the permanent magnet, the greater the current flowing through the coil. A pointer milliammeter is connected to the coil terminals, which is used as a speed indicator. Such a device allows you to increase the accuracy of readings compared to a mechanical speedometer. nine0007
The electronic speedometer does not have a mechanical connection between the speed sensor and the device in the dashboard.
The speed unit of the device contains an electronic circuit that processes the electrical pulse signal received from the speed sensor through the wires and outputs the appropriate voltage to its output. This voltage is applied to a dial milliammeter, which serves as a speed indicator. In more modern devices, the pointer is controlled by a stepper motor. nine0013 Various devices are used as a speed sensor that generate a pulsed electrical signal. Such a device can be, for example, a pulse inductive sensor or an optical pair (light emitting diode + phototransistor), in which the formation of pulses occurs due to interruption of light communication during rotation of a slotted disk mounted on a shaft.
But perhaps the most widely used are speed sensors based on the Hall effect. If you place a conductor through which a direct current flows in a magnetic field, then a transverse potential difference arises in it. When the magnetic field changes, the magnitude of the potential difference also changes. If a driving disk with a slot or ledge rotates in a magnetic field, then we get an impulse change in the transverse potential difference. The frequency of the pulses will be proportional to the speed of rotation of the master disk. nine0007
A digital display is sometimes used instead of a pointer to display speed. However, the constantly changing numbers on the speedometer are somewhat worse for the driver than the smooth movement of the arrow. If you enter a delay, then the instantaneous speed may not be displayed quite accurately, especially during acceleration or deceleration. Therefore, analog pointers still prevail in speedometers.
Despite the constant technological progress in the automotive industry, many note that the accuracy of speedometer readings remains not very high. And this is not the fruit of the overactive imagination of individual drivers. A small error is deliberately laid down by manufacturers already in the manufacture of devices. Moreover, this error is always in the big direction, in order to exclude situations when, under the influence of various factors, the speedometer readings will be lower than the actual speed of the car. This is done so that the driver does not accidentally exceed the speed, guided by incorrect values on the device. In addition to ensuring safety, manufacturers also pursue their own interest - they seek to exclude lawsuits from disgruntled motorists who received a fine or got into an accident due to false speedometer readings. nine0013 The error of speedometers is usually non-linear.
It is close to zero at about 60 km/h and gradually increases with speed. At a speed of 200 km / h, the error can reach up to 10 percent.
Other factors, such as those associated with speed sensors, also affect the accuracy of the readings. This is especially true of mechanical speedometers, in which the gears gradually wear out.
Often, the owners of the cars themselves introduce an additional error by installing tires whose size differs from the nominal one. The fact is that the sensor counts the revolutions of the gearbox output shaft, which are proportional to the revolutions of the wheels. But with a reduced tire diameter, the car will travel a shorter distance in one revolution of the wheel than with tires of a nominal size. And this means that the speedometer will show a 2 ... 3 percent overestimated speed compared to the real one. Driving with underinflated tires will have the same effect. Installing tires with an increased diameter, on the contrary, will cause an underestimation of the speedometer readings.
nine0013 The error may turn out to be completely unacceptable if, instead of a regular one, you install a speedometer that is not designed to work in this particular car model. This must be taken into account if it becomes necessary to replace a defective device.
An odometer is used to measure the distance travelled. It should not be confused with the speedometer. In fact, these are two different devices, which are often combined in one case. This is explained by the fact that both devices, as a rule, use the same sensor. nine0013 In the case of using a flexible shaft as a drive, the transmission of rotation to the input shaft of the odometer is carried out through a gearbox with a large gear ratio - from 600 to 1700. Previously, a worm gear was used, with which gears with numbers rotated. In modern analog odometers, the rotation of the wheels is controlled by stepper motors.
Increasingly, you can find devices in which the vehicle's mileage is displayed digitally on a liquid crystal display.
