NO MORE SPEEDO ERROR
Recalibrating truck’s speed sensors after tire upgrades is easy to do on 2005-older pickups with the Superlift TruSpeed Recalibrator
By Tom Morr
Swapping out factory rubber for taller (or shorter) tires affects trucks’ speed-sensor circuits, which can lead to errors in the speedometer readings and everything related to mileage figures from maintenance logs to fuel economy calculaltions and lease miles. Speedo errors can also lead to traffic tickets, which no company owner wants to see.
Most pickup owners know that tire height affects speedometer readings, but few realize that speed signals also impact anti-lock brakes, auto-trans shift points, fuel delivery, timing and other emissions-related functions.
Recalibrating the speed signal when running non-stock-height tires and/or a different ring-and-pinion ratio restores powertrain functions to their factory-engineered parameters. It also dials in correct speedo and odometer information. Avoiding speeding tickets in the most-obvious benefit. Maintenance, resale and warranty issues are also affected: Taller-than-stock tires cause the odometer to register fewer miles than actually traveled, skewing proper maintenance intervals.
On the flip side, smaller rubber will make the odometer spin faster than it should, potentially lowering resale value, prematurely passing warranty periods and incurring excess-mileage penalties in leased vehicles.
One solution to these problems was invented in Iceland. Called TruSpeed, it modifies the pulse waves between the speed signal’s source (such as the ABS sensor-circuit) and the truck’s PCM. Each quarter-turn of the TruSpeed’s dial changes the speed signal by 1% for highly accurate calibration. The unit also stores settings for two different tire sizes: winter studs, for example.
TruSpeed installation uses four wires. Although the unit’s U.S. distributor, Superlift Suspension Systems, recommends professional installation, the job can realistically be done by anyone with wiring and soldering skills.
Calibration can be the most time-consuming part of the job. The TruSpeed instructions provide approximate settings based on the difference between new and stock tire diameters and/or axle gears.
If this information is known, fine-tuning the unit is relatively fast. Calibration should be verified by using roadside milemarkers, a GPS, a radar system or by pacing with another vehicle.
Installation/calibration on this 2002 Chevy Duramax took a couple hours since we guesstimated at the long-gone stock tires’ actual rolling diameter, making our initial dial-in somewhat inaccurate.
TruSpeed applications are available for many 1992-2005 Chevy/GMC, Dodge, Ford, Jeep and Toyota trucks. (Later-model trucks with four-wheel ABS and traction control rely on a speed signal from each wheel. These vehicles can be re-calibrated through the vehicle’s OBD-II port using a feature common to many aftermarket engine re-programmer boxes.)
The gallery below shows how TruSpeed is setup. — Pro