John D Wilson from Earth on November 22, 2011: Hi Au fait. Manufacturers are guided by a standard set by the Society of Automotive Engineers known as J1226 Electric Speedometer Specification. Doing the calculations using the above equation: We can see that the car’s speedometer is reading a little slow, but since it is off by just over 2 mph it’s probably nothing to worry about. A. Reasons why your speedometer is inaccurate. New ADR 18 says speedos cannot indicate less that the true speed, and over-indication accuracy is limited to a maximum 10 per cent plus 4km/h. You can use nearly any freeway, or limited access expressway, or even a less traveled state or county road. . then use the Gear Ratio calculator, it's probably a little more accurate. Q. Speedometer gear has twenty teeth. If you know specific information about your vehicle (gear ratios, RPM, etc.) Along with the traditional functions as the speed measurement and the classical representations of the round speedometer in the car, this app will also become your trip computer, counting on how many miles you have driven in the current trip, how many hours … Whether you have a digital or manual speedometer, if it's not displaying accurate information, it doesn't really help you. BC, via email. How to test your speedometer There are two ways. I recently installed wheels and tires on my pickup with a larger circumference and used a GPS to check the speed reading on the speedometer. Speedometer Check (in MPH) Orig Tire Dia. You can look in the phone book; you can just call a couple of different auto body shops, but what you will need to do is take the vehicle in and get it certified on a specific kind of machine. Q&A - Definition of Obstructed Lane Needed. Installing larger rims and tires corrected the builtin error. - Asked by car owners everywhere. I asked Transport Canada what the standards were for speedometer accuracy. Where better than average road users satisfy their curiosity. In this particular case, let’s say that your speedometer was reading 55 mph. At 70mph the difference between the two can be as much as 5mph. There, no speedometer is allowed to read slower than the actual speed the car is going. There were no other cars around to estimate which speed is correct, but I have never had a problem before. It’s when a speedometer is off by 5 mph or more that you would may want to consider having it checked. When I am going the same speed as everyone else it often reads around 90-100mph. She said she was going about 5 MPH and the speedometer was reading 80. When taken out on a road test, the app accurately reflected the speed on my vehicle’s speedometer. But Is My Speedometer Working Correctly? Speedometer Scandal! 2. Many things can throw off your speed or the way your speedometer reads it. The car takes the recorded rotations, applies a healthy dose of mathematics and spits out a speed. Much of it is on computers far away—sometimes across the country or in another country. Speed Tracker: GPS Speedometer. It's my guess they do it this way so they meet both US and UK regulations with one standard. Steve Duckett: Well, if you're charged with speeding and you think that your speedometer might not be accurate, you need to verify that before you go to court. These numbers are the number of revolutions the speedometer cable must turn in one mile of travel. The response was short and succinct: "Transport Canada does not regulate the accuracy of speedometers. Off by 30%. GPS, on the other hand, uses a purely mathematical technique to measure the speed of the vehicle and is therefore more accurate than a speedometer. 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