Have you changed your tyre size or final-drive gear? That makes your speedo and odometer incorrect. Here’s how to get them both right.
If you’ve ever modified a car with smaller or larger tyres or, perhaps, a different final-drive gear, you’ve immediately screwed up your speedometer. This is, of course, annoying, but for modern cars with a digital speed sensor, there’s a relatively easy way to fix it. Our pal Jason Fenske at Engineering Explained shows you how in this new video, using his own Honda S2000 as an example.
This particular fix should work for any car with an electronic speedometer—so, not your ’67 Camaro with a gauge driven by a cable coming off the transmission. You’ll need to buy a speedometer recalibration device to perform this fix–Fenske uses one called Yellow Box, but you can find other options to suit your application.
Fixing your inaccurate speedometer will also require some real-world testing to determine the percentage of error introduced by your new tyres or gearing. For that, you’ll need a GPS-connected device to verify speed–a phone app, a portable navigation system, or something like a VBox should do the trick.
Once you’ve got a percentage to work with, plug the calibration device into the speed sensor–in Fenske’s case, attached to the transmission–and follow the instructions to program in the correct amount of compensation. As an added benefit, this fix will also make your odometer accurate, since it uses the same sensor that feeds the speedometer.
You’ll need to be a little handy to make this work, but the end result is worth it—and it’s way easier than the old days, where you’d have to fiddle with speedometer gears and hope you did the math correctly. After all, you don’t want to get pulled over for speeding when your car tells you you’re doing the speed limit.