Odometer correction factor - Fuelly Forums

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Old 03-18-2014, 07:51 AM   #1
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Odometer correction factor

I have a few motorcycles that are pretty close to each other in MPG. However I know that some of the speedometers / odometers are more accurate than others. It would be nice if there was a correction field in Fuelly where I can enter a number to multiply the mileage by.

E.g. if I track 10 miles via GPS, or mile markers on a highway, but my odometer clicks off 10.8 miles, I would enter "0.926" to adjust my incorrect odometer to the actual mileage, producing more accurate - and more comparable - MPGs. All mileage numbers for that vehicle would be multiplied by this correction before calculating MPG.

Many vehicle speedometers read high (I think manufacturers err toward a high reading for liability). Tire wear and inflation can have some affect on the ratio, but in general I find 5%-7% error to be common.
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Old 03-19-2014, 09:57 AM   #2
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I would agree, it seems that all of my vehicles, except for the new Corolla, are all FAST. It seems that even using the GPS show that the speedometers are showing 62 mph when indeed they are going 60 mph. That has to mean that the odometers are off, too. It would be nice to have a correction factor that we could set ourselves to do it for us every time we fill up, it couldn't be that hard, could it?
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Old 03-19-2014, 03:39 PM   #3
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The speedometer reading fast does not mean the odometer must read fast (except when they read faster than before because of a gearing or tire size change). In a modern car it's all operated by a computer program which can display whatever it wants individually on each gauge. Even without that there are plenty of ways to tune a speedometer to read fast while letting the odometer be accurate.

To check your odometer's accuracy, on the longest stretch of highway you normally drive compare mile markers, GPS distance, and your car's odometer.

However, if you're trying to compare your fuel economy to anyone else's, keep in mind that most people are just putting in the number straight from their odometer.
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Old 03-27-2014, 01:38 PM   #4
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True - speedometers tend to run fast, according to some car makers, because they want you to think you're driving faster than you are and slow down (for safety reasons, they claim). My car tends to travel 5 mph faster than actual speeds on the highway, when comparing the car speedometer to a GPS or other device. However, if the odometer is not working properly, that is a separate problem altogether.
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Old 03-28-2014, 12:48 AM   #5
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Quote:
Originally Posted by theholycow View Post
To check your odometer's accuracy, on the longest stretch of highway you normally drive compare mile markers, GPS distance, and your car's odometer.
I do it time to time to check the validity my correction, which is +2% in my bike Teresa's case. Last time I checked she was 2.5% too modest, so I'm pretty much on the safe side.

We have 2 bikes and there was always some difference between them. And one day I got Teresa's tires replaced and the difference grew too big to ignore when comparing the two bike's fuel economy, I've been using correction in my logs since then.

I used correction for my car too for a while. When I bought the YARDIS, I found the tripmeter 3-4% enthusiastic - this is how I found that the tires' size was off. After I replaced them to factory size, I stopped using correction as this enthusiasm fell under 1%.

Quote:
However, if you're trying to compare your fuel economy to anyone else's, keep in mind that most people are just putting in the number straight from their odometer.
Thankfully that 2% difference is mostly not even visible in the averages (1 digit fraction, l/100km) - I just like to be accurate

On a side note, I'm kind of developing a logging system that has this correction built-in for guys like me, and its visibility can be turned off for the ones who aren't ("kind of" = I'm learning via doing so and I'm slow)

I like to see what the odometer would show (it's already in my spreadsheet logs) if it was accurate (plus I didn't lose those kms when the decompressor didn't kick in when restarting after FAS, stole the juice from the dashboard making it revert to the last "saved game" - the last key on...)
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Old 04-28-2014, 07:48 AM   #6
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Though my speedo on my ninja 650 is WAY optimistic about my speed, the odometer is pretty close to dead on. The percent error of the speedometer is much larger than the percent error on any bike's odometer. If you really want to be accurate, add a bicycle speedometer/ odometer. Since you enter in the exact measured circumference of the tire, they are extremely accurate. You can buy correction devices for many motorcycle electronic speedometers, but way to expensive for me to care.
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Old 05-05-2014, 02:07 PM   #7
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Manufacturers have an ulterior motive as to why speedos/odos tend to read "fast". It lets the warranty run out quicker.
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Old 05-07-2014, 08:25 AM   #8
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Apple to Oranges, but my 96 Ford Bronco has a way to enter calibration numbers to correct the speedometer by a combination of pressing certain buttons while turning the key on - blah blah blah. No special programmer needed. I would think if one looks deep enough, there should be a way to do that on other vehicles too. I spent way too many years with speedometers too fast or two slow.
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Old 05-07-2014, 10:57 AM   #9
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Unfortunately it is not common to have such an easy way to adjust speedometer/odometer.

GMs from the early 1990s had a circuit board that a brave person could mess with.
http://www.tbichips.com/drac/
I imagine other manufacturers had similar designs in the same vintage. Before that there were mechanical speedometers; you merely had to replace little plastic gears mounted to the transmission where the speedometer cable attaches.

For more modern vehicles it usually requires manufacturer-specific equipment that connects to the OBDII port, such as a Tech 2 or EFILive for GM or VCDS for VW.
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Old 05-29-2014, 04:49 PM   #10
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I have calibrated my Odometers and Speedometers of my vehicles using multiple GPS units (mobile phone, TomTom, Garmin, etc.) I have a pretty good handle on the Odometer accuracy (or inaccuracy as the case may be). It would be useful to program an odometer calibration factor as a parameter for a vehicle. This calibration factor could be applied to the trip data used in the fuel economy calculations to normalize the data and cause more accurate calculations.

It's pretty bad for my motorcycle because I use a SpeedOHealer in my motorcycle to calibrate the speedometer. The side effect is that the odometer reads slightly low now. It would be great to have a
calibration factor applied to my motorcycle data.

Unfortunately I have emailed Paul about this but my suggestion was rebuffed.
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