Most speedometers are inaccurate, but is GPS better? – Jalopnik

The inaccuracy of the speedometers used by most vehicles, even those in cars with the coolest gauge cluster, is well known. The good news is that most speedometers report speeds that are higher than the actual speed the vehicle is traveling at. This is so that inconsistencies in tire size, whether due to tire pressure, tire wear or installing a different tire, do not cause drivers to exceed posted speed limits without realizing it.

GPS, short for Global Positioning System, uses a network of more than 30 satellites, allowing the GPS receiver in your phone or car to determine its position on the Earth’s surface in three dimensions. Your GPS receiver calculates changes in distance over time to determine your speed through the landscape. Although the speed indicated by GPS is generally more accurate than the speedometer in most cars, it is not without its flaws.

When a vehicle is stationary, the speedometer reads zero, as it should. However, in areas with poor GPS signal strength – under bridges and overpasses, in tunnels, or on roads with many trees overhead – the device may experience GPS fluctuations, causing the stationary device to report slow speeds. GeoTab.

Modern GPS signals have an average accuracy error rate of “less than or equal to 0.006 meters (about 0.24 inches) per second over a 3-second interval, with a 95% probability,” according to the US government GPS website. Such accuracy is good for locating your GPS within a 5 meter radius, but the mapping software that uses the signal can still send you on a GPS route over a collapsed bridge.

One of the most common ways vehicle owners make their speedometers less accurate is to install tires that are significantly larger or smaller than the standard size. While it doesn’t matter much whether the new tires selected for an off-road vehicle are mud or all-terrain tires, changing the outside diameter of the tire will cause your speedometer to register relatively different speeds than before.

For example, switching from the stock 275/70R18 tires with an outside diameter of about 33.2 inches on my 2018 Ram 3500 pickup to 35×12.5×20 tires with an outside diameter of 35 inches resulted in a speedometer reading about 2 mph slower than the GPS (and roadside speed traps) at 65 mph. Conversely, installing a smaller diameter tire causes the tire to rotate faster at a given speed compared to a larger standard tire, resulting in higher speedometer indicated speeds.

Changing the gear ratio of the vehicle’s final drive causes similar inaccuracies in the speedometer. However, outside the circles of off-road and racing enthusiasts, it is unusual for anyone to do such things.

If you find that you need higher accuracy from your GPS, consider switching to a device with dual-frequency receivers, using an augmentation system, or combining these two approaches. However, if you test the world’s fastest-accelerating electric car in an abandoned Victorian tunnel, nothing is likely to help, as GPS signals perform best with an unobstructed view of the sky.

Garmin states that the advantage of dual-frequency or multi-band GPS lies in its ability to detect (and reject) reflected satellite signals. Under ideal conditions, using a multi-band GPS device can result in location accuracy to within 6 feet. Although widely available for civilian use, the popularity of dual-frequency GPS receivers was once limited by their relatively higher cost and larger size. In recent years, high-end smartphones and some GPS-enabled watches have been using dual-frequency receivers for improved accuracy.

Augmentation systems rely on correcting GPS signal errors using fixed ground receivers and/or other satellite and radio signals. Airports and other entities, such as the U.S. Coast Guard, use such networks across the country for aviation, search and rescue, and agricultural uses. A ground-based GPS augmentation system uses differential navigation to collect GPS signals at a stationary base station, calculate any location differences it detects, and broadcast the corrections to GPS receivers within range.