Your odometer deciphered: All you will ever want to know about how it work

Dear Baraza,

Thank you for the good work you are doing. I have a question that would like your input. Why do some vehicles have a leading zero "0" on their mileage readings? Say for example 064,709? Is this normal or does it mean the first digit of the mileage could have been reset from either "1" or "2" back "0"?

Kindly advise accordingly.

Much appreciated.

Griff

Hi Griff,

Well, the reason the leading zero is there is numerical, or mathematical to say the least. Now, I could channel the mathematician in me and remind you of primary school maths in general, and the topic of "Significant Figures" in particular, but that would be long-winded and boring. A shorter answer is: the zero is used as place holder while the vehicle mileage is still below 100,000km.

Majority of road cars live a six-figure mileage life, it's only on very rare occasions where you'll find them dying out before hitting at least 200,000km, which is way more than 100,000, which, in turn, is the point of transition from five digits to six. As such, the manufacturers had* to factor this in when building the instrument clusters of their products, giving an odometer with a six-figure allowance. So, given that the odometer has six slots, what happens between the moment the vehicle leaves the factory and the point it gets to 99,999km? You will see a whole lot of zeroes, that's what.

[*: the use of the past tense here - in the word "had" - is deliberate because most manufacturers nowadays make digital odometers, not analog. With a digital display, you can write almost any number comfortably without having to add zeroes to fill in the "allocated slots"]

The vehicle will have its odometer reading 000000 while on the factory floor, then graduate to 000001 (1km mileage) as it starts moving around. It will get to 000010 (10km mileage), keep counting to 000100, and on to 001000, 010000 until 099999 (99,999km), at which point all those numerically useless but mechanically useful zero prefixes finally lose purpose and yield position to digits of real value as the vehicle mileage hits 100,000km (or miles, as may be the case for British and American market vehicles).

Now, I had mentioned that most road cars live a six-figure mileage existence, meaning that they will reach the end of their productive lives anywhere between 100,000 and 999,999km (or miles), so they don't really need to count to a million. However, once in a while we get some extremely high mileage examples creeping out of the woodwork, cars that have exceeded a million kilometers of driving throughout their use. How do you express a million kilometers, which numerically occupies seven spaces, on a six-space cluster?

You don't. The odometer will go back to zero and start counting again from scratch, and by "go back to zero", I mean it will show "000000". The owner/driver knows it's at a million kilometers, so any figure displayed beyond this point will have an extra million kilometers appended to it to get the vehicle's true mileage. This is how (and why):

The mechanical odometer is made of a series of interlocking circular drums connected either by tiny little cogs or pulleys or by a small peg that triggers the rotation of the adjacent drum. Each drum is divided into 10 segments, labeled serially from 0 to 9, and each drum gives one slot on the odometer. While we have said that the cluster gives six-figure mileages only, there are actually seven drums and this is how they are connected:

On the extreme right we have drum 1, which is almost always coloured differently from the other six, and on the extreme right we have drum 7, with drums 2 to 6 lying in between - think of the drum number layout as 7-6-5-4-3-2-1. Drum 1, the right-hand off-color drum is also divided into 10 segments, and indicates fractions of a kilometer. So, at complete zero mileage, the cluster will show seven zeroes, but the actual numerical figure is 000000.0 (take note of the decimal point).

Once the vehicle starts moving, the drum 1 starts rotating as well. It will rotate according to the mileage covered from 0.1km all the way to 0.9km at which point it triggers (via peg or gear) the immediate adjacent drum (drum 2) to the left to move up one click, from 0 to 1, as drum 1 rotates from 9 back to zero.

As you keep driving, drum 1 keeps rotating through its fractions of a kilometer to trigger drum 2 through 1km of mileage all the way to 9km. After 9.9km, drum 2 triggers drum 3 to move from 0 to 1, while drum 2 goes back to 0. Drum 1 is still rotating, so the mileage will be 10.0km.

I hope you are following. The rotations continue until 99.9km when drum3 rotates back to zero trigger drum 4 to rotate from 0 to 1, at 100.0km; and so on and so on until all the drums reach the segment 9, which is at 999,999.99km. Another 0.1km and you will be at a million at which point all the drum segments rotate back to 0 and the odometer shows 000000.0km.

In a nutshell, let's put it this way: the drums start rotating from the extreme right. Ten rotations of drum one equal one rotation of drum two (0.1km x 10 = 1.0km). Ten rotations of drum two equal one rotation of drum three (1.0km X 10 = 10.0km), and so on and so forth all the way to ten rotations of drum six equaling one rotation of drum seven (10,000.0km x 10 = 100,000.0). Ten rotations of drum 7 means a million kilometers (100,000.0km x 10 = 1,000,000.0 but there is no space for the leading digit - 1 - so only the zeroes are displayed).

***

Now comes the second part of your query: is it possible that the leading digit has been doctored downwards, such that, say, an actual mileage of 164,709(.0) or 264,709(.0) instead shows 064,709(.0). Yes, it is possible; not just possible, but probable as well.

The main problem with mechanical odometers is the ease with which they can be doctored; and this ease makes the probability of a car having its mileage tampered with extremely high. Majority of cars out there, especially with mechanical odometers, are not showing their true mileages, for obvious reasons: to fool buyers. The methods of tampering with a mechanical odometer are actually very easy.

Open up the instrument cluster of the vehicle and remove the odometer. It looks like the innards of those rubber stamps with adjustable dates on them; or if you are old enough to have used a Stagecoach bus, it looks a bit like the ticketing machine the bus conductors strapped to their shoulders like handbags (someone tells me Kenya Bus Management operations still use them, but you get my point). Once you have this equipment in your hands, there are two ways of completing your dishonorable task.

Method 1 involves a high level of craftsmanship and carries a high level of risk. You can take apart the entire odometer assembly by hand, then rotate the drums yourself to whatever number you want displayed. The risk and craftsmanship aspect comes in from the assembly itself: it is not as simple as it sounds, some joinery could be machine-effected meaning they require great quantities of either force or accuracy to take apart and/or reassemble. That in turn means you could easily break something, or have crooked results readily spotted from a mile away after reassembly.

Method 2 is foolproof; all you need is a machine or device with a spinning wheel that rotates at very high rpm, such as an electric drill, a grinder/sander, or even an electric egg whisk. Since from the description above drum one rotates the other drums in numerical order, it therefore follows that the rotating joint is attached to drum one, not drum seven. Attach your spinning device to drum one using some kind of belt, ensuring the setup rotates in the direction opposite to the one drum one rotates during normal operation. Put the hammer down and rev that electrical device until it overheats.

If your electric device rotates at, say, 5000rpm, on a 1:1 setup that means drum one is rotating 5000 times in a minute. 5000 rotations of drum one, at 10 rotations per kilometer, means you are turning the mileage backward at the rate of 500km per minute. Do this for an hour and the mileage drops by 30,000km. Three and a half hours' work gives you a mileage drop of 105,000km; or in the case of your example, from 164,709.0 down to 059,709.0.

This is just for the sake of example. If you understand classical physics you can adjust your pulley ratios between the electrical device and drum one to a 1:2 or 1:3 set up, which cuts down your work time to half or even one third of the indicated time above, meaning with a big enough or fast enough sanding wheel and a small enough drum one pulley, you can actually shave off 100,000km of mileage in a matter of minutes. You won't have to disassemble anything.

Engine management system

This is where digital clusters and odometers come in; the ones without useless zeroes preceding the mileage figure. Since the mileage is recorded electronically, you can't just remove the odometer and plug it onto a grinder and start wheezing away, lopping off the miles; because the digital cluster is just a little screen like the one found on a pocket calculator. This makes it harder to tamper with... but not impossible.

Most of the electronic data in a car is stored somewhere centrally, either in a chip or within the engine management system (ECU) itself. Accessing the ECU used to be difficult, but is nowadays easily achievable via an OBD II diagnostic connection. While accessing the ECU is now possible, changing the details therein isn't quite so easy; in some cases it's flat out impossible.

The degree of hacking involved is not only just monumental, it also carries a risk. Some manufacturers either lock away certain ECU elements via proprietary software which only they possess, while others install a digital kill switch that shuts down the whole ECU if those elements are accessed, rendering the entire vehicle useless.

There is another way round the digital odometer, one that I currently happen to have in my BMW at the moment, but it is not deliberate, it's actually a problem that I need rectified: the removal of the speedometer cable/switch from the gearbox or diff - the location varies from car to car; most cars have it in the gearbox, my BMW has it in the diff. Disconnection or removal of this cable/switch deactivates the speedometer (hence the name) but I unwittingly discovered that it also deactivates the odometer.

When mine failed, I thought I was smart enough to calculate my road speed using the prevailing gear number versus engine speed in rpm; case in point: I do 105km/h in 5th gear at 2800rpm so all my arithmetic is based off of those figures. Problem circumvented.

It was only after a long trip, while trying to calculate my fuel economy, that I realised the odometer wasn't moving either, and this was a huge problem. How do I approximate my range based on the fuel I have onboard? How do I know when to service the car? The switch went missing some years back, when the odometer was at 210,000 miles (338,000km) so I'm guessing by now I should be well north of 400,000km; but how can I even tell? I need that speedometer cable/switch reattached pronto.

This is the method I'm referring to: if you want to con the next buyer of your digital-odometer car on mileage terms, simply disconnect the speedometer cable. It will stop counting the miles, and will therefore show a figure lower than true come time to sell...

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