GPS: Speed Recognition That Is Out Of This World


Did you know that Einstein’s Theory of Relativity can actually help get you out of a speeding ticket? While having the mustached scientist show up to your court hearing is not a possibility, his work on the relationship between space and time has laid the foundation for what we now know as Global Positioning (GPS).

How in the world can we make such a leap as to associate some mind-bending astrophysical assertions to a radar-wielding public servant? Thanks to Classic Dash and Auto Meter, the connection isn’t as far-fetched as you’d think. Speed is a quantifiable element of the distance something travels in a certain amount of time. Here in the U.S., we record that as miles per hour (mph). The units may be changed according to international law, but according to Einstein’s view, they are inextricably linked into what he called “space-time”. It was his understanding that space (distance) has three dimensions, and is matched with the fourth dimension – time.

The Science Behind Your Dashboard

So, what does that have to do with the chrome-rimmed dial residing behind glass in your car’s dashboard? Well, if your speedometer happens to be one of the new GPS-enabled systems that have come to market lately, the link is easy to see once you understand a little bit about how GPS systems work. The video below is by Classic Dash, and  helps explain the difference between mechanical, electrical, and GPS-equipped speedometers.

The physical roots of GPS were embedded when the Russians tossed up the first satellite. Well known as Sputnik, a couple of American physicists were tasked with locating this man-made object via its radio transmissions as it orbited its way around the earth. In studying its location, they noted that they had to account for whether the satellite was moving toward or away from them. Known as the “Doppler Shift”, this is the very same reason that the pitch of a car horn or any other object changes as it moves toward, past, and away from you. The faster the object moves, the more Doppler Shift is exhibited.


GPS-enabled speedometers rely on the accuracy of the space program to indicate the meager speeds we travel here on earth.

The very same principle works within our GPS navigation systems that make GPS-equipped speedometers so accurate. Today’s GPS satellite network is comprised of 24 operational satellites in various orbits above earth’s surface in what is known as the GPS constellation. Remember Einstein’s three-dimensional space combined with time as the fourth dimension? Interestingly, GPS navigation takes input from at least four GPS satellites to accurately determine an object’s speed using the GPS system. Three satellites determine the position of the object using latitude, longitude, and elevation relative to sea level. The fourth satellite determines how far the object moves in a specific amount of time.


One of the benefits of using Global Positioning is being able to locate your position. The AutoMeter gauges show this as Longitude and Latitude.

Synchronizing Your Clocks

Of course, the accuracy of your dashboard GPS speedo is only as good as the input it receives. That means the position of each satellite must be precisely known, and all of the timepieces of each satellite must be incredibly accurate and synchronized with each other. To maintain this high level of time-keeping, each spacecraft carries two rubidium and two cesium clocks. These are “Atomic” clocks in the truest sense of the word.

This high level of precision takes into account the shifting and drifting of satellites. Over time, various advancements have been implemented into the GPS system of satellites to make them more accurate and useful to those on the ground. Interestingly, it takes between 65 and 85 milliseconds for a signal to come from a GPS satellite to a receiver on earth. With this high level of accuracy, even this slight delay is accounted for when interpreting the data from the satellite.

GPS Trivia

  • The first GPS satellite was launched in 1978.
  • GPS units will report driving speeds with errors reduced to about 0.1 to 0.5 MPH.
  • Most clocks work from Coordinated Universal Time (UTC). Atomic clocks on satellites are set to GPS time (GPST). GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections that are periodically added to UTC.
  • Periodic corrections are performed to the on-board clocks of satellites to keep them synchronized with ground clocks.
  • Commerce Department regulations on self-guided missile technology keep GPS receivers from reporting speeds greater than 999 mph, or altitudes in excess of 60,000 feet.
  • GPS satellites continuously transmit their current time and position.
There are several GPS systems, (NAVSTAR) is operated by the U.S. Government. The Russians have one, as well as the Chinese. There’s even a European version of GPS. The U.S. Department of Defense makes the NAVSTAR GPS system freely available to anyone with the appropriate receiver, but also reserves the right to restrict its use when necessary. Do you need to worry that knowledge of your speed is subject to the whims of the .gov? Not really. Due to your speedo needing input from several satellites throughout the entire constellation, the larger issue is ensuring that you consistently get input from the necessary number of satellites. You can improve your chances of gaining a better signal when installing your GPS-enabled speedometer, while some other considerations are simply left up to mother nature and civil engineers.

Ups and Downs Of Space-Time Speedos

GPS speedometers and location devices need to have an unobstructed line of sight to four or more GPS satellites. Even excluding situations like tunnels or covered parking garages, GPS used in urban environments can suffer inaccuracy due to multi-path interference or even losing satellite lock.

They are not however, affected by clouds or the weather. They may be affected by your wiper blades when trying to get an initial lock if the antenna is on/near the dash.

When installing the satellite receiver, it is best to ensure that it has a clear view of the sky, and as much as is possible, that it is not covered by bodywork or dashboards. Transmitter technology has improved signal strength and availability of satellite tracking, and the latest version even uses laser tracking. But, there’s still no reason not to stack the deck in your favor when possible.


When putting spacecraft into orbit, having the proper time is essential. GPS technology taps into this accuracy for reporting both speed and current time.

While there are a few limitations of using GPS technology, there are also many benefits. Drawing on the extreme precision inherent to the GPS system, you can rest assured that the information shown on your dashboard is just as accurate. In fact, several cases have gone to court where the GPS speedometer’s documented speed was successfully used to fight speeding tickets.

This brings us to another benefit of today’s electronic speedometers – the ability to recall information after the fact. While we wouldn’t use that feature to hold a high-speed version of show and tell for any LEOs alongside the road, you don’t need to worry about black helicopters hovering overhead. These systems are merely operating from a signal they receive, there is no data transmitted to anywhere.


GPS speedometers get their information from GPS receivers like this one. They need a clear view of the sky to get signals from at least four satellites. Note the coaxial end on the wire, indicating use for GPS designated speedometer.

Whether in court or at the track, the ability to document your previous speed can be handy. We’ve seen enthusiasts using this feature to double-check their recent runs at timed speed events like the Texas Mile or the Wanna-Go-Fast events using their speedometer’s peak speed recall feature.

Other benefits available via the GPS-equipped speedometer is the accurate location (shown in Latitude and Longitude), compass heading, trip and cumulative distance, and date/time/hour meter. While not informational, users of GPS technology also benefit from not having to re-calibrate their speedometer if they ever change tire sizes, gear ratio, or any other mechanical configuration of their car. Also, the transmission’s speed sensor is now nullified, since input now comes from the sky. This is exceptionally helpful to those who upgrade the drivetrain in their car or prefer not to require calibrating speed sensors to their style of speedometer.

“With an electronic speedometer, calibration is done by the person installing the gauges. All they have to do is push the trip button, drive the vehicle for two miles and push the button again.” – Greg Wambold Classic Dash

On top of all that, enthusiasts can get their current vintage speedometer converted to electronic using the latest stepper-motor technology. Doing this means they never have to worry about bouncing indicators ever again. Running wires for a battery-operated speedometer might sound more complicated than routing a speedometer cable from the transmission to the gauge, but companies like Auto Meter eliminate that concern by supplying a dash-to-car wiring harness, making this upgrade easy to install. The wiring harness is color coded and includes a very thorough and simple-to-understand instruction manual.

Classic Dash also offers complete dash assemblies with modern gauges designed for the dash opening of your auto and if you already have an electronic speedometer in your car, they even have an upgrade module to convert almost any electronic speedometer to GPS capability. A simple four-wire hook up and placement of the antenna gives you everything you need to connect to the sky! GPS-ready systems come as a kit with both antenna and speedometer, which are connected via a coaxial cable that screws into the back of the speedo. Since conversions typically use individual wiring, they have spade or screw-on connectors that simply plug into the back of the speedometer.

This conversion module makes virtually any electronic speedometer a GPS-enabled speed indicator.

Speeding Up Your Speedo

As we have stated, accuracy of both time and distance is key to getting real-time info from the space-time continuum. But also, the refresh rate needs to be often enough that we can get accurate information about our speed to be useful throughout each segment of our driving. The unit of frequency at which GPS receivers gather information from the various satellites is known in the International System of Units as the Hertz Rate (Hz). Early GPS receivers operated on a 1Hz frequency, meaning that they received information once every second.

While that sounds like a great resolution for checking-in during your drive to the supermarket, today’s units can far eclipse that rate by verifying their positioning ten times every second, also known as a frequency of 10Hz. That can make the difference similar to the pinging sonar searching for submarines or knowing exactly where we are at each turn of the wheel. For instance, just think back to the last time you missed a turn and your GPS barked, “Recalculating.” Speed does matter!

When calibrating an electronic speedometer to GPS, the pulse rate-per-mile is what dictates the accuracy of the speedometer. This varies by manufacturer, thus the need for a one-time calibration. Once calibrated, the pulse-per-mile number appears on the Auto Meter gauge in the display.

There are also differences on how quickly your speedometer expects input from the GPS receiver. This mainly concerns those electronic speedometers that have been converted to GPS capability. These speedometers register a certain amount of input pulses-per-mile, which varies depending on the make of the speedometer. This is usually done with a sensor in close proximity to a spinning wheel with teeth. Each time a tooth passes the sensor, it registers an input pulse to the speedometer. The number of teeth per mile vary by manufacturer and can range between 8,000 and 16,000 pulses.

Both speedometers use electric stepper motors for the utmost accuracy, but the speedometer on the left is GPS-ready and uses a receiver with the coaxial cable. The one on the right is simply electronic and requires a GPS adapter-module with spade connectors to utilize GPS technology.

Once converted to GPS capability, you no longer have to worry about calibrating your speedometer to the exact configuration of your automobile, as now it will be “clocked” using the GPS constellation. According to Greg Wambold of Classic Dash, “With an electronic speedometer, calibration is done by the person installing the gauges. All they have to do is push the trip button, drive the vehicle for two miles, and push the button again.” Changing tire size, gear ratio, or any other factor is now a non-issue, as the antenna now registers its position from the GPS grid.

The Bottom Line

There are many benefits to using the GPS grid to keep our speed in check. Today’s capabilities and the fact that further improvements to both speed and clarity are on the horizon, means that more autos will utilize this feature. Don’t be surprised when the OEM manufacturers start using this technology in their offerings. With the amount of integration that current production cars entail, everything from packaging to vehicle dynamics systems could benefit. For now, you can beat them to the punch and reap all the benefits for your build, whether it be a rat-rod, land-speed racer, or anything in between. Classic Dash has brough auto enthusiasts the technology now, and you don’t even have to be an Einstein to use it!

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About the author

Andrew Bolig

Andy has been intrigued by mechanical things all of his life and enjoys tinkering with cars of all makes and ages. Finding value in style points, he can appreciate cars of all power and performance levels. Andy is an avid railfan and gets his “high” by flying remote-controlled model airplanes when time permits. He keeps his feet firmly grounded by working on his two street rods and his supercharged C4 Corvette. Whether planes, trains, motorcycles, or automobiles, Andy has immersed himself in a world driven by internal combustion.
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