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.
The vehicle's speedometer gives the driver an accurate estimate of the vehicle's speed.
Speedometers on stock vehicles are calibrated for use with vehicle aftermarket parts. Replacing spare parts, including vehicle tires, may cause speedometer calibration to fail; larger tires cause the speedometer to read slower than the actual speed of the car, while smaller tires cause the speedometer to read faster than the actual speed of the car. You can calibrate your speedometer in a few easy steps.
Content
Open the hood of the car and find the gearbox. Unscrew the speedometer cable from the transmission tailshaft and use a crescent wrench to remove the bolt holding the gearbox housing cover in place. Remove the plate to reveal two gears: a drive gear mounted inside the gearbox and a driven gear attached to the speedometer cable.
Count the number of teeth on the drive gear and write down their 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 diameter to find the tire RPM.
Multiply the vehicle RPM by the number of pinion teeth found in step 1. Multiply this value by the vehicle axle ratio. Look at the ratio of the axles of the car in the accompanying documents that came with your car. It can also be found in 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 manufacturer or your local speedometer service department and purchase the required driven gear for your vehicle. You must include the make, model and year of your vehicle to ensure you are getting the correct 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.
Attach the new driven gear to the speedometer cable. Replace the gear case cover and secure it with the bolt removed in step 1. Secure the speedometer cable in place.
Determine the test drive distance required to calibrate the speedometer. This information may be in your owner's manual or other accompanying documentation, but it is also available from the vehicle manufacturer.
Press and hold the calibration button on the speedometer, start the car and release the button.
Press the button again and walk the distance prescribed by the speedometer manufacturer.
Press the button a third time after you have traveled the specified distance. After pressing the button for the third and last time, the speedometer will be calibrated to match the size of the new tire.
rf Manufacture and supply of the Russian Federation and the CIS Very often a novice tachograph master has to deal with this issue.
Very often, when installing the tachograph , it is necessary to change the speed measurement components, the speed sensor and the speedometer. But as a rule, new speedometers with a programmed K coefficient are installed to replace analog speedometers.
After installing the speedometer, the master by entering the pin code from the secret menu can enter the speedometer programming mode and change this parameter. The pin code for the speedometer is located under the rear sealing cover. Please note that on some models of speedometers, this pin code from the manufacturer is glued to a small sticker that simply disappears over time for a number of reasons.
And so everything is fine if the speedometer is new or we got the old speedometer and found the same pin code there that makes it possible to change the K coefficient of the speedometer. But what to do if, for example, there is no pin code, it is lost, erased, or the speedometer does not have a programming function, the K coefficient is programmed into it by the manufacturer, for example, as on GAZelle panels and many others.
It's really simple, provided we know what to do. We installed tachograph , made all connections, calibrated the tachograph, we know W of tachograph , then we take the signal from contact B7 of the tachograph and feed it to the input of the speedometer, of course, due to the difference in the signal coefficients of the speedometer and the tachograph, we will see inconsistencies in the speed readings.
Fortunately, if we installed a digital tachograph with a programmed output B7 in this case, we can adjust the output coefficient of the speed signal to the speedometer according to the coefficient K speedometer factory-sewn to be calculated.
And so we start calculating K of the speedometer , we take the programmed speedometer as a basis, but on the condition that we do not know the password from it, respectively, and the K coefficient too.
To calculate the K of the speedometer, you need a tachograph programmer, which should be in the arsenal of every decent workshop, we will carry out our calculations using the TS-2010 tachograph programmer. So here I pay special attention to crooks: you cannot change the mileage in the speedometer with a tachograph programmer!
I will briefly describe our actions: in general, everything is just like in school, we have a value that we do not know K of the speedometer , in order to calculate it, we need to send a signal to the speedometer with known values of speed and coefficient K and this is us can only be done with a programmer. The formula by which we will then calculate everything on the calculator is:
And so we take the programmer and connect its output signal wire to the input of the speedometer signal wire, do not forget that the mass of the programmer and the speedometer must also be connected to one point, otherwise nothing will come of it. Why? that's another topic.
So everything is connected, we go into the Analog tachographs test mode, then the speedometer test (the names of the menu items in different programmers may vary, but I think you will figure it out)
Set the output signal parameters:
We press the Start button on the programmer and look at the speedometer needle, it begins to deviate, as can be seen in the figure, when a signal with the parameters we have set is received, the speedometer needle deviates to a value of 61 km / h.
We take a calculator and start the calculation:
We get and write down the result0003
After all our simple actions, we write this obtained coefficient into the output signal parameters of the B7 tachograph.
We get: our tachograph measures the signal from the speed sensor using its own calculations, for example, the value is 10000 imp. per km. further processes this signal and outputs it through the programmed output B7 to the speedometer, BUT already with the values that we set 4918 as a result, everything is adequately displayed and everyone is happy. But do not rush to rejoice, because. after some time of operation t.s. the client will definitely call you and inform you about the discrepancies in the mileage of the tachograph and the speedometer, this is due to the inherent error in the speedometer.
Manufacturers in the production of speedometers deliberately put in them an error that underestimates the real speed characteristic according to GOST. Moreover, this error is linear, i.e. with increasing speed t.s. the error increases. If the error were not linear, then with a more qualitative and accurate measurement of the coefficient K of the speedometer using the above method, it would be possible to achieve a minimum discrepancy, but someone will say "we measured the readings of the movement of the speed arrow by eye", and here is No. Even under the condition that we install a new speedometer with a known pin code with which we entered the same K coefficient as in the tachograph, there will be a run on the speedometer, and from experience I can say that when measuring K of the speedometer , taking into account the error (I mean the method described above), the discrepancies between the tachograph and the speedometer are much less.
I would also like to note the category of speedometers that interact with a digital tachograph via the CAN bus, i.e. these speedometers also receive information about speed from a digital tachograph BUT they do not measure the input signal, it simply is not there. Information about the speed is simply transmitted by the tachograph to the speedometer via the CAN bus. When using a combination of speed sensor - digital tachograph - CAN speedometer, the difference in the readings of the total distance traveled will be zero. The CAN speedometer is, roughly speaking, just an indicator; it does not have its own speed calculation mechanism. These speedometers are installed in European trucks; in the new MAZ KAMAZ models, it is not something that is not necessary but not possible to calibrate them. I advise you to read all the same what a CAN bus is.
Notes:
Standard K values wired from the factory in programmable speedometers:
In our experience, we used a speedometer 140mm 12V 811.
