How to fix a misreading speedometer

It’s a problem many may not know they’re experiencing until flashing lights in the rearview mirror tell them otherwise. How do you know if your vehicle is reading an accurate speed? And if it isn’t accurate, how do you go about fixing the problem?

Have no fear, because Jason Fenske from Engineering Explained is here to show us, and it’s a fairly simple diagnosis and fix. Jason uses his personal Honda S2000 as a test mule, but the process in the video can be applied to nearly any car. If you’re curious, his speedometer is off due to Jason adjusting the final drive ratio, but different wheel and tire combos can similarly throw off a speedo.

First, it’s essential to see how high or low the speedometer is reading. In the S2000’s case, it’s reading 7.4 percent too high, showing a speed of 87 mph when Jason is traveling at a GPS-verified speed of 80.4 mph.

The problem can be solved with a Yellow Box correction kit. Plugging the correction kit into the S2000’s speed sensor, which is located on the transmission, allows the Yellow Box to read the speedometer, and, when properly programmed, make the correction.

Once the Yellow Box is plugged into the transmission, turn the to accessory mode to power the Yellow Box and lights will begin to flash on the device. The flashes will tell you the current correction factor that is programmed. Out of the box, it’s 100 percent, with “1” signified by a quick flash and “0” indicated by a longer flash.

As stated, the S2000 is reading 7.4 percent too high, so the correction factor can be changed by pressing the “plus” button 74 times. After the input, the Yellow Box will again read out the correction factor you have applied. After making his changes, Jason decides to zip-tie the device to the transmission, ensuring it’s tucked away from the exhaust and other heat sources.

Hitting the road shows the correction factor has brought the S2000’s speedometer much closer to the actual speed, reading 81 mph at a GPS-verified speed of about 80.2 mph. While it is possible to fine tune the device even more, Jason is happy with the result.

Grab a closer look in the video above.

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