What causes mechanical odometer to switch from miles to kilometers?

Question

My Dodge Avenger was made in 1996, and the odometer is currently showing 270K miles, just to give you a sense of the age of the vehicle. I've been getting suspiciously high mileage for at least the last year or so. The mileage app I'm using (Fuelio, highly recommended) says my average mpg is 43, and one fill-up recently my mileage was over 51 mpg.

So I finally used the My Tracks app (which uses my phone's GPS) and compared its readings to my odometer. I did this several times, and it seems my odometer is consistently showing 1 mile for about every 0.61 actual miles I drive. For instance, on one trip my odometer showed 26.6 miles, and My Tracks said 16.2. At that ratio, my mileage is closer to 26 mpg, which sounds a lot more realistic for this car.

So what would be causing this odometer inaccuracy? Just about everything is stock, and I'm using the standard tire size.

I know this is a long shot, but any chance my odometer might actually be registering km instead of miles? The ratio is just about right (1 km = 0.62 mi). I have no idea why it would be doing that since it's a US vehicle. It's probably also relevant to mention it's a mechanical odometer, so it's not like I can press a button to switch between miles/km. I've done some googling, but I've only found one similar instance.

More details:

• The car was built and owned entirely in the US
• I purchased the car in a wrecked condition from a salvage yard and had it rebuilt
• I'm fairly certain the odometer was registering miles when I first bought the car
• To the best of my knowledge the instrument cluster was never changed
• I was having an unrelated problem at one point and changed the ECU with one I bought from a salvage yard in the US. Replacing the new ECU with the original did not correct the issue.
• The transmission was also rebuilt at one point

Answer 1

If your speedometer is accurate, you can check that with your GPS or phone as well. The only real option is the cluster (The thing with all the gauges and odometer). You can send it off to have it repaired, or get a used one from a junk yard. If you ever sell the car you are legally bound in most (if not all) US states to tell the buyer of an odometer discrepancy. Just document the current mileage when you change the cluster and what the new cluster had when you installed it. If you get your current cluster repaired then I would just make a note of the mileage when you repaired it.

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Update

Others have suggested that this normalish, but this is a major difference in mileage traveled vs mileage reported by the odometer, this is not a normal condition.

I do find it interesting that one mile = 0.621371 km, so if you speedo is correct and reading in mph, your odometer is measuring km. Now depending on where the car was manufactured the odometer may be in km and not miles. Do you have a switch to change from km to miles? Is the readout digital for the odometer?

The odometer is turned by a stepper motor which get's it's input from the PCM (Powertrain Control Module) which get's it's information from the VSS (vehicle speed sensor). The speedometer gets it's info from the PCM as well. So if the speedometer is showing you the correct speed then we know the VSS is sending the correct info to the PCM.

I see two possibilities

There is a problem with the PCM or PCM programing (software) that is causing the Odometer to read Km, causing the computer is turning the stepper motor too fast.

The wrong stepper motor or gear in the cluster has been installed, either at the factory or when a repair was done.

Since you don't know if it was ever reading correctly you don't know if it's been that way from the factory, or if a repair prior to your ownership caused the problem. If it's been that way from the factory you car has considerably less miles on it that the odometer shows. Proving that is another problem entirely.

Answer 2

While your speedometer sounds particularly inaccurate, the simple fact is that almost all car manufacturers design their speedometers to be inaccurate because of speed legislation in many countries:

From thecarexpert.co.uk, EU law says that

A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph. Or to put it another way, if your speedo is reading 50mph, you won’t be doing more than 50mph but it’s possible you might actually only be travelling at 40mph.

To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any forseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount. As your satnav is not the designated device by which a car’s speed is measured, it does not need to incorporate any fudge factoring.

Additionally, as a tire wears down, the speed shown will creep higher and higher in relation to the real speed (because the tire has a smaller diameter, it will end up spinning more times per mile, indicating a higher speed)

Answer 3

I'll go with a combination of the standard overstating of mileage on the car (well put by Rory) and also GPS inaccuracy... I've been doing Road Rally for a long time and a big part of that is precision mileage measurement. As an instrument rated pilot, I also have a strong interest in GPS navigation.

One of the things I learned when experimenting with GPS for Road Rally is that it does a couple things that cause it to understate mileage. It doesn't pick up the extra mileage from going up/down hills properly and it tends to cut corners (it'll grab a sample a little before you get to the corner and a little after and connect them with a straight line shorter than the actual mileage traveled).

GPS is great tool for figuring out where you are at a point in time. It is also great at telling you how far from some other point you are at a specific point in time. It's not a great tool for taking a whole bunch of those points and adding them up accurately (samples take time, and you can't do continuous samples without killing the battery, so they're periodic and therefore susceptible to additive error). Also, standard GPS vertical height measurement is poor due to the low angle of the GPS constellation. With WAAS support, height measurement becomes much more accurate (as it involves at least 1 high angle satellite), but not all devices/applications support WAAS (or even height measurement at all).

Answer 4

It sounds to me as though your car was imported from a country which uses KM. It may be that it's had it's instrument binnacle replaced at some point in the past with an import unit.

Typically it's fairly easy to convert a KM speedo to MPH by simply removing the perspex and needle and sticking a replacement facia marked up with MPH over the speedo. It's significantly harder to change the odometer from KMH to MPH so this may be what has happened.

There is a slim chance that the odometer supports both options and there is a switch (or jumper) hidden away on the back of the cluster which has been set incorrectly from the factory. This setting could conceivably be controlled from the ECU but I'm not familiar enough with your model of car.

Manufacturers inaccuracy is usually legislatively limited to a maximum error of 10% with most modern cars being inside a 2% error so based on your massive over-read, I'd say it's set to KM almost certainly.

Answer 5

You mentioned that transmission was worked on. Was the differential changed to a different ratio? A friend of mine years ago had a pickup truck that went into the shop reading miles per hour and came out reading in kilometers per hour. His speedometer was mechanical (pre-digital days) and was cable connected to the transmission.