Speedometer Calibrators 101
Contents of this Guide
Click a topic from the list below to skip ahead to a particular section.
- Do I Need A Speedometer Calibrator?
- Types of Speedometer Calibrators
- Important Notes
Do I Need A Speedometer Calibrator?
Now that you've upgraded your tires, you may be wondering if this will affect the accuracy of your speedometer. In short, unless your new tires are the same diameter as your old tires, yes, your speedometer is reading less accurately.
Most vehicles are programmed to convert the number of tire revolutions per minute into the miles (or kilometers) per hour displayed on your speedometer. Obviously these calculations are based upon your factory tires.
If you increase the size of your tires, your new tires must make less revolutions to cover the same distance. Your vehicle, unaware that its tires are now larger, will register the revolutions per minute and display your speed as less than your actual speed. In other words, you are traveling faster than your speedometer indicates. How much faster? Well that depends upon both the difference in your new tire's diameter and the speed at which you are traveling - the larger your new tire's diameter and the faster your rate of speed, the less accurate your speedometer. A common method to determine the extent to which your speedometer is off is to compare your vehicle's speedometer to a radar speedometer found in a nearby school zone or construction zone. No, you do not need to exceed the speed limit to determine the accuracy of your speedometer. If your speedometer is reading 1-2 MPH under a speedometer in a 35-45 MPH zone, your is likely reading 4-5 MPH under your actual speed at 60-70 MPH.
It is worth noting that changes in the size of your wheel or changes in the width of your tire do not matter. In this scenario, the accuracy of the speedometer is only affected by changes in the overall height (diameter) of your tire.
Aside from ensuring accurate readings, speedometer calibrators also correct your transmission's shift points. Just like your speedometer needs to be adjusted after installing larger (or smaller) tires, your vehicle's computer system also needs to be programmed to shift gears at different RPMs. Speedometer calibrators should also adjust your transmission's shift points to ensure a smoother ride and better performance.
Types of Speedometer Calibrators
There are 2 main types of speedometer calibrators. The type of vehicle you own may limit which option is available to you. To determine your option(s), follow the links in the Brands section below.
Standard (Handheld) Speedometer Calibrator
Standard speedometer calibrators are generally quick and simple to install (~20 minutes). These "Plug & Play" models are connected directly to a vehicle's diagnostic connector (OBD2 port). Once calibration is complete, the calibrator is disconnected from the vehicle but should be retained in case future recalibration is needed.
Some units may require simple programming/updating using a PC before being connected to the vehicle.
Inline Speedometer Calibrator
Inline speedometer calibrators can be a bit more labor intensive. The calibrator must first be connected to a PC for programming. Then, the calibrator is joined to the vehicle. Vehicle dependent, it will either be tied directly into the vehicle's instrument cluster behind the speedometer or into the body control module under the driver side dashboard. The calibrator remains permanently connected to the vehicle.
If the calibrator requires disassembly of the driver side of the dashboard, this process can take 1-2 hours in total.
- Carefully follow the speedometer calibrator manufacturer's instructions. Failure to do so may damage the device, or worse, your vehicle's computer system.
- Most speedometer calibrators must be linked directly to a specific vehicle. Once connected to a vehicle, it may not work with any other vehicle.
Check out our preferred speedometer calibrators here:
Automotive customization can be complicated. Mistakes can be costly. We highly recommend consulting an expert before purchasing or installing aftermarket accessories.
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