Slamming your truck to get it in the weeds on low-pro rubber or jacking it up for big, knobby meats will throw your speedometer out of whack. Most custom-truck builders know this, but few do anything about it. In older trucks with mechanically driven speedos, the correction involves changing the gear on the cable. This either requires a trip to a speedo shop that has mph-calibrated rollers or some at-home trial-and-error, with gears having various teeth counts. As a result, most truck owners just live with the fact that their speedos are 10 percent or more off at highway speeds.
Trucks began using electronic speed-sensor systems in the early '90s. While electronics usually mean fewer hands-on opportunities for do-it-yourselfers in general, the speed sensor isn't one of these areas. The aftermarket has developed straightforward methods for the home mechanic to recalibrate many trucks with electronic speed sensors.
An Icelandic invention called TruSpeed offers a straightforward way to correct the speed signal in popular late-model, computer-controlled trucks. TruSpeed intercepts the speed signal between its source, such as the ABS sensor circuit and the PCM, modifying the pulse waves so that the truck's computer receives accurate information. Each quarter-turn of the TruSpeed's dial changes the speed signal by 1 percent for highly accurate calibration. The unit also stores settings for two different tire sizes.
The TruSpeed kit comes complete...
The TruSpeed kit comes complete with mounting and wiring hardware, as well as a calibration key. The toggle switch on the left allows storing and selecting settings for two different tire sizes.
In addition to controlling the speedometer reading, the vehicle's speed sensor can impact other systems, such as antilock brakes, auto-trans shift points, fuel delivery, timing, and other emissions-related functions. Fixing the speed signal to register actual speed and distance traveled restores these functions to their stock parameters. Maintenance, resale, and warranty issues are also affected: Taller-than-stock tires cause the odometer to register fewer miles than actually traveled, and smaller, low-pro rubber will make the speedo spin faster than it should-prematurely voiding mileage-based warranties.
TruSpeed installation only involves four wires and can be done by anyone with basic skills who can also solder. For calibration, the TruSpeed instructions provide approximate settings based on the difference between new and stock tire diameters and axle gears. Calibration should be verified by using roadside milemarkers, a GPS, a radar system, or by pacing with another vehicle.
These photos show a TruSpeed installation on a Chevy Duramax. TruSpeed applications are available for many '92-and-later GMC/Chevy, Dodge, Ford, Jeep, and Toyota trucks. The main installation differences among vehicles are the locations of their fuse boxes and speed-sensor wires.
This leisurely installation took about 2 hours. A nearby speed-survey radar trailer helped us dial in and verify speed calibration. A few passes were necessary, since our initial "dial in" was off-this truck's stock tires were long gone and we guessed at their diameter. Using a handheld GPS is perhaps a more popular way to fine-tune the TruSpeed.
1.If possible, mount the TruSpeed...
1.If possible, mount the TruSpeed close to the fuse box. The unit is water-resistant and can even be mounted in a dry place under the hood, if necessary. On this late-model Silverado, the under-column area works well.
3.Pull the fuse with the appropriate...
3.Pull the fuse with the appropriate tool. Install the supplied tapper connector and press the fuse back into place (far left, second fuse from the top).
4.Crimp the supplied spade...
4.Crimp the supplied spade connector onto the TruSpeed's red wire. This connection goes to the tapper in the fuse box.
5.Route the TruSpeed's green...
5.Route the TruSpeed's green and yellow wires through the firewall or door-hinge area, if the wires can be protected from moving parts, and into the engine compartment to the PCM. We used a piece of wire to fish them through a firewall grommet.
6.Sensor wire locations vary...
6.Sensor wire locations vary among vehicles. This Chevy's speed-sensor wire is in the PCM, which is under the hood on the driver side. Release the mounting clips to free the PCM.
7.Refer to the TruSpeed instructions...
7.Refer to the TruSpeed instructions to isolate the correct sensor wire, and cut it.
8.Strip the factory speed-sensor...
8.Strip the factory speed-sensor wire.
9.Slide the kit's heat-shrink...
9.Slide the kit's heat-shrink tubing over the wires, then crimp-connect the TruSpeed's yellow and green wires to the truck's speed-sensor, following the instructions.
10.Solder each crimped connection...
10.Solder each crimped connection for added insurance.
11.The kit's heat-shrink tubing...
11.The kit's heat-shrink tubing provides a third line of defense. A heat gun is recommended, but a hair dryer will work in a pinch.
12.Superlift recommends wrapping...
12.Superlift recommends wrapping electrical tape over the heat-shrunk connections before returning the PCM to its mount. Then, use the kit's zip-ties to secure the green and yellow TruSpeed wires away from hot and moving parts.
13.Crimp the supplied eyelet...
13.Crimp the supplied eyelet connector onto the TruSpeed's black wire. Secure this connector using an existing bolt that's grounded to the vehicle.
14.Refer to the instructions...
14.Refer to the instructions and then use the calibration key to dial in a ballpark speed sensor signal. Fine-tune the speed signal-each quarter-turn equals 1 percent correction-using a GPS, a radar, or by pacing alongside a vehicle that has an accurate speedometer.
15.Following final calibration,...
15.Following final calibration, use the supplied hook-and-loop fastener to mount the TruSpeed unit. Secure all excess wires with kit's zip-ties and reassemble the dash.