Tag Archive for On-board Diagnostics

Why Increased Accuracy Matters

–By Chuck Lane, Solution Engineer, Webtech Wireless

With ever increasing fuel costs, a common question these days is, “how do I accurately measure my individual vehicle fuel consumption?”

GPS versus Vehicle Odometer Reporting: “It’s a mine field”

With the advent of on-board diagnostics (OBD) in the 1980s the problem was solved, right? In an automotive context, OBD is a generic term referring to a vehicle’s self-diagnostic and reporting capability. All vehicle manufacturers conform to SAE J1979, and the OBD-II standard has been mandatory for all cars and light trucks sold in the United States since 1996.

“It’s complicated”

Problem solved! We just connect to the vehicle OBD port (they all have one) and read the fuel consumption data, right? Wrong—it’s not that simple—it’s complicated and fraught with pitfalls.

Each vehicle manufacturer implements the OBD standard uniquely and will rarely, if ever, share that standard outside its service network. It’s possible to gain access to that standard and implement a solution to read the PID (parameter identification) number codes, but manufacturers are not required to implement all PIDs listed in J1979, and they are allowed to include proprietary PIDs that are not listed. It’s just a minefield to attempt to interpret individual manufacturer PIDs accurately.

Most importantly, non-approved connections to a vehicle OBD port (ECU/Engine Control Unit) could invalidate warranty and cause other legal or technical issues. Modern vehicles are controlled by highly sophisticated computer systems, can detect miniscule unexpected current draws, and may register a fault. What if you have a mixture of old and new cars, vans, buses, and other vehicles from different manufacturers? Older vehicles don’t have any OBD port at all.

Comparing OBD against GPS Statistics

Both OBD collected data and GPS calculated data have some inherent flaws—neither one is 100% accurate. Recent studies have shown a variation of plus or minus 5% for speed and odometer data from OBD. We recently conducted a series of studies with our major customers involving hundreds of vehicles measuring the accuracy of ODB versus our GPS locator information. With recent improvements in our  locator hardware and algorithms we have now confirmed our GPS information is accurate to plus or minus 3% versus OBD data from the same vehicles.

The federal specification for vehicle speed/odometer is plus or minus 10%. A recent study by a major Webtech Wireless customer has shown that their trucks were reporting a speed of 15% to 20% higher than actual when using cruise control (this data was taken from the ECU (engine control unit)). This prompted the customer to switch to GPS calculations for speed.

Odometer Drift Analysis for 827 Vehicles

Recently, Webtech Wireless completed a study of 827 vehicles from a large customer fleet. This study compared the odometer readings from the vehicle with the mileage calculations from the Webtech Wireless Locator using GPS calculations.

The results of the study are as follows:

Odometer Drift Analysis for 827 Vehicles

  • Average GPS odometer drift was plus or minus 3.07% when compared to vehicle odometer
  • 87% of the vehicles had less than 5% drift
  • 63% of the vehicles had less than 3% drift
  • 2% of vehicles had GPS antenna issues (19 vehicles)

In an additional study, one of our largest clients found ECU data on speed to be way off, so they asked us to switch to GPS calculated speed.  When the GPS antenna was properly mounted and had clearance to the sky (i.e., in highway conditions further from obstructions such as skyscrapers), GPS speed and odometer calculations were favorable.

Increasing Accuracy

The biggest factor in getting good GPS odometer (mileage) readings is to have the GPS antenna installed properly. Improper installation can cause a high number of no-fix records that, in turn, can cause invalid GPS odometer readings.

The following actions can increase GPS odometer accuracy:

  • Install a GPS antenna with a good view of the sky
  • Update the GPS odometer for all vehicles every 6 to 12 months
  • Update the GPS odometer during scheduled maintenance
  • Monitor vehicles with high no-fix rates of greater than 25%


In conclusion, both vehicle OBD and the GPS odometer readings can have some issues regarding accuracy. Webtech Wireless Locators and highly accurate GPS calculations provide proven GPS accuracy for speed and odometer readings. These studies resulted in our customers using Webtech Wireless GPS reporting for accounting purposes.