issued: august 2016 future truck program position …...

Future Truck Position Paper (2016-1) — 1© 2016, TMC/ATA

Future Truck Program Position Paper: 2016-1

Dynamic Adjustment of ‘Wheel Rotations Per Mile’ Via GPS-ECU Comparison

Developed by the Technology & Maintenance Council’s (TMC)Condition-Based Maintenance Task Force

Issued: August 2016

ABSTRACTThe ability to refine wheel-based road speed calculations through the adjustment of wheel rotations per mile via global positioning satellite (GPS) input is a desirable feature that does not yet exist for heavy-duty commerical motor vehicles (CMVs). This technology, which would dynamically adjust rotations per mile to account for tire wear, could improve speedometer ac-curacy and fuel economy reporting (as reported by the engine electronic control unit or (ECU). It also has the potential, if wheel-end sensors are employed, to report current remaining tread depth and the need for tire replacement due to wear. This technology would compare vehicle distance travelled as measured by both a GPS-enabled device and the vehicle ECU, and should a reasonably significant difference exist, the ECU’s “wheel rotations per mile” param-eter would be automatically adjusted accordingly, to compensate for tire wear.

Technology & Maintenance Council (TMC)950 N. Glebe Road • Arlington, VA 22203 • Ph: (703) 838-1776 • FAX: (703) 838-1701

[email protected] • http://tmc.trucking.org

INTRODUCTIONMost commercial vehicle fleets specify their vehicles with a maximum road speed setting, also referred to as the “governed speed.” This governed speed is generally recorded in the engine ECU, which uses computer software to keep the vehicle’s road speed from exceeding this limit. The system has its limits, of course; a tractor-trailer going down a steep grade may still require the driver to use the brakes to keep the vehicle under this limit. That being said, the ECU generally does an excellent job in enforcing the governed road speed on level

and moderate grades, especially when cruise control is in use.

The main parameter used to calculate road speed is “wheel rotations per mile,” and is generally specified by the vehicle owner and recorded electronically in the ECU.

The number of reported wheel rotations per mile at a given road speed can vary depending on the amount of remaining tire tread depth. Because of this, fleets often choose a rotation value setting that represents the 50-percent


tread life point (i.e., when half the tread is worn off). Thus, a driver whose truck has new tires can achieve an actual (otherwise known as “real” or “effective”) road speed that's slightly higher than the governed speed. Similarly, a driver whose truck has significantly worn tires can achieve an actual road speed that's slightly lower than the governed speed.

In the real world, this variance causes two is-sues. First, the ECU-provided fuel economy values are either over- or under-stated during most of the life of the tire (by as much as five percent — see Figure 1). Second, driver per-ception of attained fuel economy suffers, par-ticularly when the tire is substantially worn.

DISCUSSIONThere is typically a two-to-three MPH difference in calculated speed compared to actual speed based on tire wear (new to old). For example, using a new tire that has 20/32nds inch(1.5875 cm) of tread, the number of rotations required to move an “actual mile” is lower than after the drive tire has worn down to the legal limit of 2/32nds inch (0.15875 cm) of tread. A loss of 18/32nds of an inch of tread (0.5625 inches or 1.42875 cm) equates to a change in the tire diameter of 1.125 inches (2.8575 cm).

The circumference (C) of a circle is calculated using the formula:

C=2πr (where r is the radius of the tire), or;

C=πd (where d = diameter).

This represents a loss of 1.125 (2.8575 cm) inches of diameter and, using 3.14159265359 as the value of π, equates to a reduction in circumference of approximately 3.534 inches (8.97636 cm). For simplicity, let’s round that to a reduction of 3.5 inches (8.89 cm) of circum-ference. Restated, this represents a 3.5-inch (8.89 cm) loss of travel per rotation from the tire’s “new” condition.

For the sake of discussion, let’s presume the ECU has a programmed value of 500 rotations per mile (1.60934 km). A driver going 65 MPH (104.607 km) as per the unmodified vehicle road speed would experience 32,500 wheel rotations in one hour, potentially losing 114,864 inches (291754.56 cm), or approximately 1.81 miles (2.912913 km) of actual travel per hour between when the tire was “new” and when it was “worn to the legal limit.”

There are 63,360 inches (160934.4cm) per mile (1.609344 km). So, in this example, there is a nearly two-MPH difference (3.21869 KPH) between how far/fast a vehicle actually travels with new tires versus old tires.

Figure 1 provides an example of this variance by tread depth (1).

Availability of GPS on CMVsThe U.S. Department of Transportation (DOT) issued a new ruling in 2015 that mandates the

Figure 1(1) Source: 2008 Michelin engineering presentation sup-plementing the MICHELIN® Truck Tire Data Book.


use of electronic logging devices (ELDs) for hours of service compliance (as opposed to paper logs) in CMVs by the end of 2019. In or-der to satisfy the mandate requirements, ELDs must have access to the vehicle’s geographic position, typically achieved by means of GPS which is already heavily utilized by various navigation products in CMVs and passenger cars/light trucks. TMC’s Future Truck Committee notes that several original equipment manufacturers (OEMs) have already formed strategic alli-ances/partnerships with various telematics providers. Every major, in-cab telematics pro-vider already provides support for and access to a GPS signal.

The following links illustrate this capability (links are provided are examples only; no endorse-ment should be implied):

• http://fleetowner.com/technology/peo-plenet-supply-telematics-kenworth-peterbilt

• http://fleetowner.com/technology/volvo-adds-optional-telogis-solutions-its-own-connected-vehicle-platform

• http://telematicswire.net/navistar-to-install-rand-mcnally-intelliroute-in-its-trucks/

GPS Signal AccuracyAccording to the official U.S. Government website about GPS — http://www.gps.gov — real-world data from the Federal Aviation Administration (FAA) shows that high-quality GPS standard positioning service (SPS) receiv-ers provide better than 3.5 meter horizontal accuracy (see http://www.gps.gov/systems/gps/performance/accuracy/).

There is some truth to the common belief that civilian GPS signals are less accurate than those used by the military. According to gps.gov, “the accuracy of the GPS signal in space is actually the same for both the civilian GPS service and the military GPS pulse per second

or PPS service. However, SPS broadcasts on one frequency, while PPS uses two. This means military users can perform ionospheric correction, a technique that reduces radio deg-radation caused by the Earth’s atmosphere. With less degradation, PPS provides better accuracy than the basic SPS.”

Thus, commerical users have 137.8 inches (3.5 meters) of GPS signal inaccuracy to ad-dress. This is not “per mile.” It is a level of inaccuracy for any one given point providing a maximum inaccuracy of 275 inches (698.5 cm) between the start of a trip segment (key-on [or perhaps motion detected]) and the end of a trip segment (key-off [or perhaps road speed drops to zero]).

To minimize the impact of this inaccuracy, TMC believes the ECU-computed distance traveled should be compared to the GPS-computed distance traveled for a trip segment only at increments of five or greater miles (8.0467km), producing a difference (delta) between the two sources (ECU and GPS).

Accordingly, if any segment is less than five miles, it would be discarded as too short. If greater than five miles, then the distance calculation could either be performed once, at the end of the segment, or at a maximum frequency of every five miles (or such other minimum distance as determined through testing). Testing may subsequently determine whether a shorter segment is appropriate.

Perhaps there should be a requirement of ac-cepting a GPS measure-of-travel only when engine load is between “x-and-y.” This would address the potential impact of uphill and down-hill grades influencing the GPS-travel-distance reliability. However, GPS does offer the ability to determine “altitude” so that hilly terrain would not necessarily be of general concern. And, perhaps there should be a requirement that multiple trip segments be used to confirm the delta value. Experimentation would certainly


provide a consistent and reliable “comparative distance” factor. Once the delta value is com-puted, it would be then used to determine the actual circumference of the tire and to compute the adjustment to the rotations per mile that would then be used by the ECU.

TMC’s Future Truck Committee notes that at least one OEM (Cummins) already provides a manual-entry field within certain ECU and engine models that accomplishes a rotation adjustment (see Figures 2 and 3).(2) This is commendable and deserves recognition.

However, the manually projected/entered adjustment value still leaves some degree of inaccuracy (because it relies upon “projected”

tire wear), and creates the need for “touching the truck,” to maintain accuracy — albeit less frequently than other models that lack this adjustment value. Certainly, the ECU would have to be “touched” any time the drive tires are replaced. Failing to reset the values could result in a truck being able to vastly exceed the intended governed road speed.

Another element to consider, even when using the manually entered rotation adjustment field, is that the adjustment factor and remaining tread values need to be changed each time tires are changed, and the “adjustment of wear per thousand miles” used when running the original tires may change significantly with a tire change. For example, what if the original

Figure 2

Figure 3

(2) From the CM2350 Electronic Subsystem Technical Package — OEM Programming Guide AEB 15.141

(2) From the CM2350 Electronic Subsystem Technical Package — OEM Programming Guide AEB 15.141


tire was “Brand X” but was changed to “Brand Y”, with completely different wear data? How would this data be gathered and subsequently shared/updated across the country, with every site that may replace a tire?

These challenges serve to further emphasize the value of this proposed technology, as the equipment in vehicle itself would “indirectly inform” the ECU of actual tire circumference (rotations per mile) on a continuing basis.

NOTE: It is “indirect” in that the GPS data is for distance traveled which is then used by the ECU to calculate the tire circumference.)

The value of more accurate ECU-reported fuel economy data that would result from adop-tion of this technology is self-evident, as this impacts purchasing decisions and decisions as to selecting equipment for trade-in or sale. As equipment ages, it comes on the radar for disposal (as in trade-in or sale). The “final” selection decision may be influenced by the reported fuel economy of groups of trucks with similar characteristics such as make, model year, engine, transmission, gearing, etc. Trucks with significant age and/or high odometer miles would typically be the ones “up” for consideration for decommissioning and would be expected to have tires that are worn closer to the legal limits. This means we would expect those trucks to be overstating their fuel economy.

Let’s examine this by way of an admittedly over-simplified example. A truck with worn tires reporting (via the ECU) to have traveled 48,000 miles and to have burned 8,000 gallons of fuel (six MPG) has potentially only traveled a geographic distance of 46,560 miles (assuming a delta of three percent) and is actually experi-encing a fuel economy of 5.82 MPG. If we also use this data to determine which new trucks to purchase as replacements, the purchase deci-sion we make may not have been the correct one, compounding the impact of inaccurate

data. Additionally, a reduction in fuel economy is often interpreted to mean the vehicle is due for maintenance which may or may not actu-ally be true, and could lead to increased cost associated with bringing a truck into a shop or taking it temporarily out of service.

One must also consider the frequency at which the ECU would alter the rotational value used to compute road speed — probably, no more frequently than once per week. If this limita-tion is implemented, however, there should probably be a parameter available at the ECU or on the telematics device that resets this proposed frequency-timer, to be used when a tire change has indeed taken place. We note that frequent “writes” to at least some forms of flash memory may be ill-advised(3).

Also, how large and how small a “delta” should be used to alter the value used to compute road speed? For example, a delta of less than x rotations per mile would be discarded as in-significant and a delta greater than y rotations per mile is unlikely to be accurate and should be discarded. Certainly a sensor failure or a tone-wheel sensor (magnet) out of adjustment could/would lead to unrealistic changes in data. If a reasonability-check is indeed implemented, there should probably be a parameter available at the ECU or on the telematics device that temporarily (for x thousand miles/kilometers) overrides the reasonability check’s limits, used when a tire change has indeed taken place. Although – by discarding any update that is based on a reading of less than X number of miles, we would allow an update for tires go-ing from 2/32 to a lug type tire or a tire size change without having to reprogram the ECU by hand.

SAE J1939 already has a number of suspect parameter numbers (SPNs) (i.e., J1939 fault codes) defined that fit right in, including:

(3) Jonathan Thatcher, Fusion-io; Tom Coughlin, Coughlin Associ-ates; Jim Handy, Objective-Analysis; Neal Ekker, Texas Memory Systems (April 2009), “NAND Flash State Storage for the Enterprise, An In-depth Look at Reliability.”


• Altitude• Latitude/Longitude• GPS Speed

In order to assist equipment users by giving them a greater level of confidence in the process, should there be an new SPN for GPS signal quality defined to report the change via the databus in a manner similar to the fault that is used to report the expiration of the idle timer that then causes an engine shutdown? If so, should the fault content include the delta value?

It would be required if the ECU relies upon a telematics provider to supply the GPS data. It would be self-evident to the ECU if the GPS signal were inherently available through the addition of its own GPS antenna and circuitry which is probably a cost-effective alternative to defining and implementing an interface to any telematics system. This technology ensure the accuracy of the speedometer and odometer is improved over what is currently available.

Communication ConsiderationsWhile there are many communication methods used on CMVs, the vast majority have some form of system provided by a third-party telem-atics provider that includes a GPS component. Since the ELD mandate requires that GPS data be available and used as an input to the elec-tronic driver log; and, the same regulation also determines the frequency of GPS availability to the application, information from that GPS component can and should be provided ro the ECU, possibly via the J1939 databus. (TMC’s S.12 Onboard Vehicle Electronics Study Group

should consider developing a recommended practice on this subject.)

In addition to latitude and longitude, TMC suggests the standard-defined data include a GPS signal quality rating as well as some form of GPS-calculated miles driven. The ECU already knows what kind of engine load has been occurring, but a standard would be needed to define what engine load ranges would be accepted (perhaps a parameter within the ECU) to validate a trip segment so that severe grades would be excluded from data used for delta calculations. This exclusion would not be necessary if the GPS calculation of travel takes “altitude” into account. Altitude is available whenever the GPS chipset has a lock on four or more GPS satellites.

In any event, the data could be evaluated on a regular basis, such as once every two seconds. The ECU would be provided the data “upon request” provided a standard is defined. Such a standard would be expected to specify that:

• The telematics product would provide the information upon request from the ECU.

• The structure passed to the ECU would only need to record the GPS signal quality for the initial, starting point of the a trip segment and for the terminating point of that same trip segment. (See Figure 4 for an example of the possible fields, although the example does not yet cor-respond to the trip segment rules.)

• The telematics product would store (and pass upon request) the “most recently

Figure 4


completed trip segment” while actively accumulating the “current” segment. Upon completion of a trip segment, the newly completed segment would then replace the “most recently completed trip segment.”

• The “current” trip segment would end automatically when the ignition key is cycled off, and would be discarded if it is under five miles (8.0467km) in length.

Safety ConsiderationsDriver attention is very important to the safe operation of vehicles. There are many products being produced to supply information to the driver. The question should be asked, “does the driver need to know when or if an adjustment is adopted or incorporated into the ECU logic?” TMC does not feel this is needed; however, we wish to note the question was discussed.

Need for Additional StandardsIndustry must develop meaningful standards for increasing the accuracy of distance traveled and road-speed calculations. Such standards to be reviewed and potentially / hopefully

implemented by both, engine manufacturers as well as 3rd part telematics providers. TMC recommends the standards be developed through the joint efforts of S.12, S.2 Tire & Wheel and S.3 Engine Study Groups. The Society of Automative Engineers (SAE) may need to be involved to define a new SPN and/or fault code structure or content.

There are existing standards that may be applicable/functional. The following SPNs/parameter group numbres (PGNs) are already defined and a part of the J1939 protocol. (See Figure 5.) As stated previously, only an SPN for GPS signal quality is missing.

Summary of Requested ActionsTMC’s Future Truck Committee recommends that the Council and/or industry develop rec-ommended practices involving:

• Field testing (or, at a minimum, lab testing) to verify the feasibility of this proposal.

• Data elements to be gathered.• The manner of presentation of the data

elements (e.g., decimal accuracy).

Figure 5 (continued on next page)


Figure 5 (continued fron previous page)


• The appropriate minimum (and perhaps maximum) length of a valid/acceptable trip segment.

• The manner and structure of data ex-change from the telematics provider to the ECU (which would be unnecessary if the GPS circuitry were directly available to the ECU).

• The frequency or conditions under which the ECU would be expected to request/work with the data structure (e.g., key on, or transitioning to 55 MPH from a lower speed, for example.)

• The “tolerance” for the delta value before the ECU would use it to alter the value used to calculate the road speed. Some would be rejected if out of range (fails to pass a reasonability check). An override in the event of a tire change is probably appropriate.

• The maximum frequency that the ECU would alter the value used to calculate the road speed. (e.g., once per timeframe or once per x miles/kilometers). A reset process used in the event of a tire change seems appropriate.

• The manner of notification to the vehicle owner (fleet) of a revision to the value used to calculate the road speed, if said notification is deemed appropriate (such as [possibly] through a fault code).

TMC’s Future Truck committee notes the U.S. DOT is, and has been, considering a require-ment that the maximum governed road speed of a truck will be set by the factory such that it cannot exceed a specific, yet to be determined, maximum road speed. Without this proposed technology implemented prior to that mandate, drivers could end up operating substantially (as much as five percent — see Figure 1) below the desired governed maximum speed as they approach the 2/32nds minimum legal

tread, if the governed speed's rotations per mile is set in the ECM based on the installed "new tire" circumference. With this in mind, TMC asks that ATA’s advocacy team take this into consideration and, perhaps, provide DOT officials this position paper while the regulation is still not yet published.

Additionally, TMC’s Future Truck Committee notes one of the industry’s common methods of driver compensation is “pay by the mile.” As such, the vast majority of those paid by the mile are paid “by the book mile” rather than “by the dash odometer” mile. The minority of drivers who may be paid by the dash odometer mile should see no long-term or significant impact with the implementation of this technology. The dash odometer will simply reflect a more accurate number of miles traveled at any point in time. Over the life of the tire, the current ECU recording/reporting method and this proposed method will report the same number of dash odometer miles (presuming the fleet uses the mid-point tire circumference). The difference lies in the fact that the current ECU recording/reporting method would show the driver ran fewer miles on a given route when the truck is equipped with new tires than when that same truck runs the same route using worn tires. Phrased another way, absent this proposed technology, the paycheck of a driver paid by the dash-odometer miles traveled is understated as to dash odometer miles with new tires and overstated with worn tires. That inequity is re-moved with the implementation of this proposed technology. For those (TMC believes few) fleets both using the original “new tire” circumference for determining wheel rotations per mile and paying the driver by the dash mile, they may continue the practice through the installation of a hubometer set as they wish.

issued: august 2016 future truck program position …...

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