It’s quite common for us to swap tire sizes and go with bigger rubber on vehicles. Once you do that though, the speed signal coming from the transmission or transfer case will no longer display a speedometer value that matches the actual speed you’re moving. Larger tires make the displayed readings less than the actual speed.
A simple example of how much a tire change can affect your readings is easy to illustrate. If you start with a common truck tire of LT225/75R16 (about 29 inches tall) and swap that up to an LT285/75R16 (about 33 inches tall) you’ll see a speed reading error of about 14 percent.
In the old days, this only affected your speedometer display and your odometer accuracy. However, on vehicles that now use on-board computers, an electronic speed sensor is used to tell electronic control modules your moving speed in order to adjust fuel mixture, transmission shifting, etc. So when swapping to a tire size that is appreciably different than the stock size, it may be beneficial to correct the speed sensor readings.
Some late model vehicles use one speed sensor to drive the speedometer, while another may provide input to the electronic modules to affect shifting and engine control. In this case, a little more work may be involved to understand the system.
You can easily check how accurate your speedometer is by using a GPS unit to display your moving speed and comparing that to your vehicle reading. An alternative is to match your odometer readings with road mile markers over some good distance and calculate the error from there.
In any case, today there are a number of solutions to get your speedometer back to displaying the correct speed. It’s just a matter of determining what type of sensing system you have and investigating available products. Here are some examples:
Many domestic transmissions and transfer cases have the ability to accept different OEM gears for the various factory gearing and tire configurations. Often, these can be swapped to recalibrate your rig for the gears/tires you are running. A number of Dana/Spicer/New Process cases use this method and the gear drive electronic sensor shown here is one from a Jeep application being used in an Advance Adapters Atlas transfer case.
In older applications where a mechanical cable travels from a gear drive in the drivetrain up to the instrument cluster, speedometer ratio adapters may be available that will work. These are essentially units that fit in the mechanical cable path and change the cable speed with a gear converter. Some are available off-the-shelf and others have to be custom built with the appropriate mating ends.
Today, OBD (on-board diagnostics) compatible monitors and engine tuners include tire recalibration, and they can display corrected speed readings. If you want to correct the readings going to the engine control module and the factory speedometer cluster you might consider using a module such as this Abbott Electronic Ratio Adapter (ERA). This small unit provides the ability to choose a multiplier to account for any combination of gear ratios and tire size to provide an accurate speedometer readout. The ERA accepts a pulsed signal from the factory speed sensor and modifies it to the correct pulse rate needed for the speedometer or engine control module.