Most Speedometers Are Inaccurate, But Is GPS Any Better?

The inaccuracy of the speedometers used by most vehicles, even those in cars with the coolest gauge clusters, is well known. The good news is that most speedometers report speeds higher than the actual speed the vehicle is traveling. This is so that inconsistencies in tire size, whether due to tire inflation pressure, tire wear, or installing a different tire, won't cause drivers to exceed posted speed limits without being aware.

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

When a vehicle is stopped, its speedometer will read zero, as it should. However, in areas with poor GPS signal strength –- under bridges and overpasses, inside tunnels, or on roadways with heavy tree cover overhead –- the unit could experience GPS wobble, causing the stationary unit to indicate low speeds, according to GeoTab.

Modern GPS signals have an average accuracy error rate "less than or equal to 0.006 meters (about 0.24 inches) per second over any 3-second interval, with 95% probability," per the U.S. Government's GPS website. Accuracy like that is good for locating your GPS within a 16-foot radius, but the mapping software using the signal could 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 installing tires that are significantly larger or smaller than the stock size. While it doesn't make much difference if the new tires selected for an off-road vehicle are mud terrain or all-terrain tires, changing the outer diameter of the tire will cause your speedometer to register relatively different speeds than before.

For example, changing from the stock 275/70R18 tires with an approximate outer diameter of 33.2 inches on my 2018 Ram 3500 pickup to 35x12.5x20 tires with a 35-inch outer diameter resulted in the speedometer reading approximately 2 mph slower than the GPS (and roadside speed traps) at 65 mph. Conversely, installing a smaller diameter tire causes the tire to spin faster at a given speed compared to a larger stock tire, resulting in higher speedometer indicated speeds.

Changing the final drive gear ratio of the vehicle causes similar speedometer inaccuracies. However, outside of off-road or racing enthusiast circles, it's uncommon to find anyone doing such things.

If you find yourself needing higher accuracy from your GPS, consider switching to a unit with dual-frequency receivers, using an augmentation system, or combining those two approaches. However, if you're testing the world's fastest accelerating EV in an abandoned Victorian tunnel, nothing is likely to help, since GPS signals perform best with an unobstructed view of the sky.

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

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