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Author Topic: Errors in speedometer and trip meter  (Read 1073 times)
dickreid1
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« on: August 06, 2012, 07:39:48 pm »

Well I just turned 10,000 miles on the odometer however it is actually more like 10,120 miles.

From the beginning it was obvious that the E450 speedometer was lower than the GPS and traffic radars; over 1 mph but less than 2 mph.  The difference remains the same at all speeds.

At each fillup I find the Odometer/Tripmeter readings of the E 450 are also short of the miles logged by a Scangauge calibrated to the GPS.  Over the fillups so far this summer the average error is 1.2%.  That means the actual distance traveled averages 12 miles farther for every 1,000 miles shown on the odometer.

This represents  0.1 increase in miles per gallon and a small extemsion in the Ford warranty.
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Bob Mahon
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« Reply #1 on: August 07, 2012, 09:57:06 am »

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 Im 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 Odometers indicated mileage. 

A question comes to mind here; why cant 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.
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« Reply #2 on: August 07, 2012, 11:20:18 am »

Like Bob M., the speedometer in my 2010 E450 reads higher by about 2 MPH at 60 MPH compared to the GPS.  In fact, all three of our vehicles read high and I also assumed that it was intentional.  Yes, tire wear is another error factor causing the speedometer to read higher, and to confuse the situation more, I have found that different tire brands may have different new diameters for the same tire size.

Regarding the Scangage, yes, it did also read higher than the GPS but it has a selectable correction factor that allows it to be set to read the same as the GPS.  I will need to reset it as the tires wear.

Bob A.
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TomHanlon
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« Reply #3 on: August 07, 2012, 12:30:32 pm »

Is that with or without the radio on? 
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Tom Hanlon
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« Reply #4 on: August 07, 2012, 12:49:42 pm »

Tom,
With the radio on and the windows down, the drop in mileage is exactly 0.4673256847. thinking
With just the radio, it's 0.1837574632 (accounting for the HP requirement added to spin the Alternator).
Unless it's a CD, which I have no info on. shrug
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« Reply #5 on: August 07, 2012, 01:23:30 pm »

Before you guys need to get calculators with more digits of readout, you need to go read up on terms like accuracy, precision, repeatability, reproducabilty, random error, and significant figures.  I was once in an honors geometry class in high school where a guy with a real photographic memory recited pi to a couple of hundred places.  The teacher wisely gave him a zero for wasting his time.

Jerry
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Bob Mahon
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« Reply #6 on: August 07, 2012, 02:00:18 pm »

Not being in a High School Geometry class, we Engineers always work to finite values in order to satisfy the requirements and avoid personal liability. Wink
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« Reply #7 on: August 07, 2012, 04:13:26 pm »

OK Bob are you taking into account the what is playing. I would be sure my mileage goes way down when I listen to 50's music over a talk radio station. Now hill billy music makes me drive really fast, just trying to get away from it. rolling on the floor
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Tom Hanlon
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« Reply #8 on: August 07, 2012, 05:15:42 pm »

Don't know 'bout that but Hip-Hop makes me drive like I'm dodging spear ----err, Lions.
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« Reply #9 on: August 07, 2012, 05:17:43 pm »

Paul says the problem is the unusually high atmospheric temperatures. We have this problem in Texas all the time! The roads actually expand in length and the GPS has no way to know this, so using its old memorized maps it gives the wrong speed calculation. That's why the rest of the country is experiencing the problem this year! It's hot everywhere.

Thanks for an entertaining afternoon! rolling on the floor
Marilyn
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Bob Mahon
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« Reply #10 on: August 07, 2012, 06:15:53 pm »

Marilyn,
That's a classic and I'll remember it for the next time this comes up. Cheers
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« Reply #11 on: August 07, 2012, 09:14:51 pm »

I'm turning the radio off!     rolling on the floor
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« Reply #12 on: August 27, 2012, 10:28:18 pm »

Oh to be retired and have this kind of time, but alas work and teenagers are calling....

Live well, laugh often, love much.

DJM
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