What Is Better: a Mechanical or an Electric Speedometer?

Modern electronic speedometers are generally superior in every factor but cost.

Speedometers are one of those things that drivers look at every day, but rarely think about unless something goes wrong. Cable-driven spedometers have been serving reliably for over a century, but have recently fallen out of favor for the more sophisticated electronic type. Some of this preference has to do with the electronic speedo's inherent benefits, but much of it is due simply to the fact that it's easier and cheaper to install in a computer-controlled vehicle.

How Speedometers Work

Cable-driven and electronic speedometers aren't as different as you might think. A cable-driven or "eddy-current" speedometer uses a cable connected to the transmission output shaft to spin a small magnet inside the speedometer. That magnet spins inside of a spring-loaded aluminum "cup" attached to the speedo needle; the faster the magnet spins, the more it drags the needle. An electronic speedometer also uses magnets, but in this case the magnet is stationary and on the transmission case. A gear-like wheel on the output shaft spins in front of the magnet, inducing a pulsating electric current that another "Hall Effect" sensor reads. A computer interprets that signal and uses the information to move a servo attached to the back of the speedo needle.

Mounting

Either configuration permits mounting pretty much anywhere in relation to the transmission, but the eddy-current speedo's cable offers limited flexibility. Unlike the electronic unit's wires, a cable cannot traverse 90-degree angles without a bit of sideways twist. It's not an insurmountable obstacle in most cases, but often times chassis originally designed for an electronic speedometer won't provide enough transmission tunnel clearance to accept a mechanical speedometer cable. Realistically, though, if you're retrofitting an engine and cable-drive transmission into a vehicle designed solely for electronics, then cable fitment is probably the least of your worries.

Accuracy

An electronic speedometer's Hall Effect sensor and servo arrangement is about as close to perfect -- at least in terms of accuracy -- as is practically possible. This configuration is the same one used to run the engine's control computer and the car's anti-lock braking system; in fact, many new cars use the ABS sensors to detect wheel speed in lieu of a separate transmission-mounted sensor. These sensors register upwards of a dozen times per revolution -- thousands of times per second. The cable in a mechanical speedometer is prone to stretching and twisting over time, which may cause the unit to wear out faster and deliver less immediate and precise results.

Cost

The mechanical speedometer's primary benefit is that it's cheaper to produce and install. All those sensors and processors cost money, and a spinning cable and magnet are cheap. But even this isn't true all the time, particularly in vehicles that already use Hall effect sensors and computers to control the ABS. In cases like this, the speedometer is just one more thing plugged into the computer.

Adjustability

Increasing or decreasing a vehicle's tire size or final drive ratio will affect engine rpm relative to road speed. A larger tire travels more distance per revolution, which will cause the speedometer to read at a lower speed than the vehicle is actually traveling; smaller tires will cause the speedo to read high. It is possible to adjust a mechanical speedometer by changing the gears that connect it to the transmission output shaft, but this may require tearing into the transmission and still may not yield a 100-percent accurate reading. Adjusting a computer-controlled electronic speedometer is as simple as plugging a compatible computer into the speedo's control unit and changing the settings.

References

• "How to Diagnose and Repair Automotive Electrical Systems; Tracy Martin; 2005
• "Undertanding Automotive Electronics"; William Robbins; 2002
• "Foundations of Electrical Engineering"; John Cogdell; 1995