How to check the accuracy of your speedometer, Imposing a 30 km/h Speed Limit in Residential Areas. Older cars will need a different speedometer drive gear installed in the transmission. This means that at a true speed of 100 kilometres per hour the speedo can’t be displaying 99, or under, but it could be displaying up to 114. First, does your vehicle have original specification wheels and tires? - Car and Driver Magazine. You can use nearly any freeway, or limited access expressway, or even a less traveled state or county road. Therefore, 5 minutes and 15 seconds would be 5 + 15/60 minutes, or 5.25 minutes. Sky Drive is the world’s most accurate electronic speedometer sending unit. Speed Tracker: GPS Speedometer. Speedometer: Aftermarket modifications, such as different tire and wheel sizes or a change in the differential gearing, can cause speedometer inaccuracy. However, it can go up to 10% above the actual speed itself. Where better than average road users satisfy their curiosity. Often, the best way to do this is to have your speedometer calibrated by a qualified professional, followed by a certificate from that professional that verifies the accuracy of your speedometer. "If my sat nav shows I'm going 4/5 miles an hour slower than my car speedometer says, which do I believe?" Manufacturers are guided by a standard set by the Society of Automotive Engineers known as J1226 Electric Speedometer Specification. If the speedometer reading is lower than the actual speed, the speedometer is slow. Businesses know that proximity makes a difference, and that’s why some, like Netflix, use a content delivery network to bring the data closer to you. Ah, yes. Image credit: Pixabay. Probably rear … At 70mph the difference between the two can be as much as 5mph. The trouble is that for virtually all of these incidents, I knew how accurate my speed measuring device was but these drivers had no idea whether their speedometer was accurate or not. Whether you have a 1969 Chevrolet Nova Super Sport, a 50s era Oldsmobile Rocket Eighty-Eight, or even a British sports car like a Jaguar E-Type the speedometers basically work the same. When taken out on a road test, the app accurately reflected the speed on my vehicle’s speedometer. If you notice that your vehicle’s speedometer doesn’t seem to give you an accurate reading or is not working at all, then you have an issue on your hands that a mechanic may need to remedy- unless you are a mechanic. This could be due to two reasons. To prevent automated spam submissions leave this field empty. Speedometers are not allowed to read lower than the actual speed of the car, but they are permitted to indicate a value that is 10% higher than the actual speed, plus 4. then use the Gear Ratio calculator, it's probably a little more accurate. I had read on a forum dedicated to that particular make of truck the speedometers usually read 5-7 kilometres slower that the actual speed. So, chances are if you are looking at 70mph on your car's speedometer, your actual speed is probably closer to 67mph. The car takes the recorded rotations, applies a healthy dose of mathematics and spits out a speed. My Speedometer Is Off, How Do I Figure Out What My Actual Speed Is There was one of those highway department signs that said “your speed is”. Yes, the mechanics can adjust the computer that controls your speedometer to take into account changes in tires/wheels, a change in gearing or something similar. On the occasions I use my smartphone as a sat nav, I get a readout of the speed of the car on the screen. Much of it is on computers far away—sometimes across the country or in another country. How to change your car’s engine oil and filter, How to clean salt and grime off the car after winter, Haynes shows you how on YouTube: Nissan Pathfinder, How to save money on car maintenance when you’re a student, 21 Nissan Versa videos added to the Haynes YouTube channel. 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