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I've always heard that way-back-when, the simplest way to rewind a car's odometer was to drive backwards (at least in some models). Is this true? Did those manufacturers miss such an obvious gaming method?

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    What a classic from Ferris Bueller -- perhaps the most famous example... (LINK) – Hendy Jun 27 '11 at 23:12
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    wiki says true but no refs, only way to be fully sure is to ask a mechanic that worked on those and tried it. OTOH I recall the movie Mathilda showed the father forging an odometer by turning the cable backwards with a drill (how accurate that is depends on how well hollywood researched that) – ratchet freak Jun 27 '11 at 23:28
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    It was also mentioned in the Roald Dahl book Matilda. – Andrew Grimm Jun 28 '11 at 11:40
  • @Andrew +1, I was going to mention the same. ;-) Loved that book. – Konrad Rudolph Jun 28 '11 at 11:42
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    Driving backwards a few thousand miles still does not seem like the simplest method ... ever tried this on public roads - not very practical! – Daniel Jun 6 '18 at 12:53
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Yes, older cars used mechanical odometers, which go forward or backwards, depending on which way the gears are turned. Modern cars use electronic odometers. I couldn't find anything indicating over what time period this changed. It was well before my time behind the wheel, though.

I also found a January 1961 article from Popular Science Magazine on the prevalence of odometer rollback fraud. (Page 59, if the link doesn't jump right to it)

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    There was an intermediate period with mechanical odometers that (a) wouldn't go backwards when in reverse and (b) were tamper-evident for rollback. I can't find a good reference. Plenty of anecdote though. – Oddthinking Jun 28 '11 at 0:34
  • no word if any safety (like a simple one way slip lock or an adapted coaster brake) is used on the output on the gearbox to the odometer – ratchet freak Jun 28 '11 at 0:37
  • @Ratchet, I think such devices were added (in 70s and 80s) but I can't find proof. – Oddthinking Jun 29 '11 at 8:44
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    Indeed, whether the odometer is mechanical or electronic has nothing to do with it; either it is built to not go backwards, or it isn't. – psusi Jul 1 '11 at 15:40
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    Not all mechanical odometers ran backwards, vintage Ferrari's never did, all odometers (USA) would not reverse starting in the 80's to prevent fraud. – Moab Aug 25 '11 at 1:38
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Mechanical odometers could be tampered with in myriad ways. I have seen people literally use a screwdriver to manually move the numbers backwards, I have seen drills used and various other methods. Even now, you can have your 'mileage adjusted' digitally; it's a huge area of crime here in the UK and people are still getting jailed for it.

Odometer

This link actually mentions Ferris Buellers Day off:

You can also see that mechanical odometers like this one are rewindable. When you run the car in reverse, the odometer actually can go backwards -- it's just a gear train. In the movie "Ferris Bueller's Day Off," in the scene where they have the car up on blocks with the wheels spinning in reverse -- that should've worked! In real life, the odometer would've turned back. Another trick is to hook the odometer's cable up to a drill and run it backwards to rewind the miles.

How stuff works

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    It was a 61 Ferrari 250 GT Californian; it would have defintely had a mechanical Odometer – Hairy Jun 28 '11 at 12:20
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    @Hairy: how do we know it "definitely" would have had a mechanical, reversible odometer? You seem to have experience... but how are the rest of us to know this? – Hendy Jun 28 '11 at 15:44
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    @Hairy: for example, HERE is a granted US Patent from 1935 with several mentions of using ratcheting gears to make various parts non-reversible... – Hendy Jun 28 '11 at 15:52
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    @Hairy, part of Hendy's point is that while you know you've seen it done, and you know what would be in a '61 Ferrari, we don't know that you know. If you can provide a reference, it will probably shut us up. – Oddthinking Jun 29 '11 at 8:47
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    I know Veglias, of that era and up to 1979, were mechanical speedos, with mechanical odomoeters. I know there were no rachet patents applied to them, I know, and have proven, you can adjust them (mechanical odometers) by drill, screwdriver, driving backwards, etc. However, I do not know, nor can I find, the precise Veglia instrumentation model number, a 1961 Ferrari 250 Californian used. However, the car in the film, was a kit car based on an old MGB and even had Smiths instruments (one close up shows this), which were mechanical, and I have adjusted these. – Hairy Jun 30 '11 at 6:47
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A point which no one has yet brought up is where the odometer cable is attached. If it is attached after the gearbox, then it will go forwards or backwards. If it is attached before the gearbox, then no matter whether you travel forwards or backwards, it will always move in a positive direction.

According to this article on madehow.com, speedometers attached to the back of the transmission started to be used from 1921 (prior to this they were attached to one of the wheel axles, which would allow for reversal of the mechanism)

So Hairy's point that mechanical odometers are automatically susceptible to reversal is not the case. I can't find any definite evidence for the percentage split between wheel attachments and engine attachments, though, but as @Oddthinking pointed out, it has been made moot anyway with the introduction of digital odometers.

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    If it was attached before the gearbox, it will count revs of the motor, not of the wheel: The ODO would show same speed and distance when driving at 2000RPM in first gear as in last gear, though real speed (and distance) may differ by a factor of about 4 or more. So, it does not make sense to attach the ODO before the gearbox. It makes sense to attach it directly behind the gearbox, because distance to the dashboard is shorter, and because the gearbox doesn't move so much w.r.t. the chassis. Maybe, you confuse gearbox & differential? The input to diff. also reverses its direction... – sweber Mar 10 '16 at 9:00

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