My calculations based on Speedometer and GPS indications and conclusions differ somewhat.
When running at 60 MPH by Speedometer, the GPS displays 58 MPH, which indicates the Speedo is reading high. As 60/58=1.0345 (103.45%), the Odometer will register 103.45 miles for every 100 miles traveled.
Regarding MPG:
If we use this 1.0345 correction factor, the calculated MPG must be divided by it to arrive at a more accurate MPG value. Example: An initial value calculated using 10 gallons of fuel to travel 100 miles would indicate the unit is getting 10 MPG. Using the correction factor of 1.0345 as a divisor, a corrected MPG value is 9.67 MPG.
Regarding warranty period:
Presuming the Odometer and Speedometer are in identical proportion, the 36,000 miles warranty will expire at only 34,799 true miles (36,000/1.0345=34,799.42).
Let me introduce another factor:
Our Speedometers/Odometers register miles based on pulses a Hall sensor in the transmission generates and transmission rotations are in direct proportion/relation to wheel revolutions. Any aftermarket device, such as a Scangage, Ultragage or the like, will also be dependent on the signals transmitted by the Hall sensor. This can be presumed to be the only constant.
The tire size on our PC units is 225/75R16, which, when new, is a diameter of 29.29”, equating to circumference of 92”, resulting in 7.67 feet/revolution. Consequently, our tire/wheel must rotate 688.7 times per mile.
As the tire wears, the diameter will reduce by as much as 0.4” per side, making the diameter 28.49”, equating to a circumference of 89.5”, resulting in 7.46 feet/revolution. Consequently, our now worn tire/wheel must rotate 707.7 times per mile. Overall wear results in a value of 1.0276 (707.7/688.67=1.0276).
So again we have a factor that will negatively impact the accuracy of Speedometers/Odometers and aftermarket products.
The point I’m trying to make is that, regardless of what you use to calculate MPG, constantly changing factors will impact economy and any warranty will expire early by being based on the Odometer’s indicated mileage.
A question comes to mind here; why can’t the auto manufacturers install a more accurate Speedometer/Odometer in this day and age of sophisticated electronics? I believe it to be a liability issue. Almost universally, regardless of brand, Speedometers generally read high. Consequently, a person ticketed for speeding cannot reasonably seek remedy from an automaker based on a low reading speedometer.
