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#1
Old 03-29-2011, 07:11 PM
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Why doesn't driving a car in reverse gear reverse the odometer?

A car in reverse gear does not make the odometer go backward. I had thought the rotation of the tires was the measure by which an odometer calculates mileage, but, if so, how does going in reverse still add miles to the odometer? Is something other than tire rotation being measured? Or is a device taking the absolute value of the rotation as the measure?
#2
Old 03-29-2011, 07:40 PM
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I don't know if car odometers work on the same principle, but on my bicycle the odometer/speedometer is triggered by a magnet passing a sensor once with each revolution of the wheel. The sensor doesn't know or care which direction the magnet is passing it, it just registers a pulse as the magnet passes. So the odometer works with the wheel spinning in either direction.

On looking up how car speedos work, it's also based on magnets, albeit a different method. Again, though, it seems all that matters is the speed, not the direction.

Last edited by Colophon; 03-29-2011 at 07:40 PM.
#3
Old 03-29-2011, 07:42 PM
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Quote:
Originally Posted by GlennRayinTroy View Post
A car in reverse gear does not make the odometer go backward.
This was the case with mechanical odometers, but that made it easy to tamper with them. (See this pretty cool look at the inside of an odometer.)

Modern odometers are electronic. It uses a sensor within the transmission -- either magnetic or optical -- to measure each rotation of gears, which translates to rotation of wheel and thus a measure of movement.

ETA: colophon is correct. the difference between bike and car is that the car odometer does not have a sensor on the wheel, but rather in the transmission. Otherwise, the principle is the same.

Last edited by anson2995; 03-29-2011 at 07:43 PM.
#4
Old 03-29-2011, 07:56 PM
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So I guess the Ferris Bueller scene with the Ferarri was wrong.
#5
Old 03-29-2011, 08:02 PM
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Even mechanical odometers had a ratchet mechanism preventing reversing. I know, because I tried it on a rental car ( I used a drill to spin the odo pickup cable in reverse).
#6
Old 03-29-2011, 08:27 PM
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Quote:
Originally Posted by Bijou Drains View Post
So I guess the Ferris Bueller scene with the Ferarri was wrong.
Nope. I was spot on accurate.





Their attempt failed to reverse the odometer.
#7
Old 03-29-2011, 08:35 PM
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Quote:
Originally Posted by Bijou Drains View Post
So I guess the Ferris Bueller scene with the Ferarri was wrong.
Actually, in the movie, their attempt to set the odometer back didn't work. IIRC, that was what set Cameron off when he kicked the car and sent it through the back of the garage.

ETA: Or, in other words, what Philster said (I need to learn how to type faster, dangit).

Last edited by engineer_comp_geek; 03-29-2011 at 08:36 PM.
#8
Old 03-29-2011, 08:37 PM
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But they were dumb enough to leave the car on the jack for all that time without checking to see if what they did worked.
#9
Old 03-29-2011, 08:59 PM
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Quote:
Originally Posted by Bijou Drains View Post
But they were dumb enough to leave the car on the jack for all that time without checking to see if what they did worked.
And they were dumb enough to think it would work....which it didn't. I don't get your point.
#10
Old 03-29-2011, 09:34 PM
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In fact at one time it did work. Not sure how a mechanical one could be kept from reversing it.
#11
Old 03-29-2011, 10:05 PM
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Speedo/odometers with a cable drive usually run off a gear in the transmission, so they don't care which way the wheels are turning. Since the engine only turns one way, so does the front of the transmission. Some cars do it differently, but it's a lot of trouble to hook the speedo to the front wheel, what with steering and suspension and all. The trans never moves. If the car is relatively new, you can run the speedo off the ABS sensors in any wheel through the computer, but then the above-mentioned "magnet" theory is in play.
#12
Old 03-29-2011, 10:15 PM
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Quote:
Originally Posted by thelabdude View Post
In fact at one time it did work. Not sure how a mechanical one could be kept from reversing it.
Engineers are clever.
#13
Old 03-29-2011, 10:43 PM
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Quote:
Originally Posted by engineer_comp_geek View Post
Actually, in the movie, their attempt to set the odometer back didn't work. IIRC, that was what set Cameron off when he kicked the car and sent it through the back of the garage.
Geez, spoiler alert!
#14
Old 03-29-2011, 11:07 PM
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Air-cooled VW beetles most certainly did run their speedometers off the front left wheel. May be true for other air-cooled VWs.
#15
Old 03-29-2011, 11:28 PM
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Quote:
Originally Posted by andyleonard View Post
Speedo/odometers with a cable drive usually run off a gear in the transmission, so they don't care which way the wheels are turning. Since the engine only turns one way, so does the front of the transmission.
That doesn't make sense. If you ran it off the engine output / transmission input you'd only be counting engine revolutions. The distance travelled depends on the actual gear being used at the time, i.e. the number of transmission output / wheel revolutions.
#16
Old 03-29-2011, 11:47 PM
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Quote:
Originally Posted by andyleonard View Post
Speedo/odometers with a cable drive usually run off a gear in the transmission, so they don't care which way the wheels are turning. Since the engine only turns one way, so does the front of the transmission.
Uh, no.

Quote:
Originally Posted by andrewm View Post
That doesn't make sense. If you ran it off the engine output / transmission input you'd only be counting engine revolutions. The distance travelled depends on the actual gear being used at the time, i.e. the number of transmission output / wheel revolutions.
Right. The speedo signal is not taken from the front or input of the tranny, it's taken from the output shaft. The cable spins in one direction going forward and the opposite direction when reversing. The odometer is designed to only register in the forward direction.
#17
Old 03-30-2011, 07:42 AM
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Umm... this is getting interesting:

http://auto.howstuffworks.com/car-dr.../odometer1.htm

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.

While that does work on older mechanical odometers, it does not work on the new electronic ones


If anything, there are great photos of the mechanical inner workings.... along with the interesting conclusion...
#18
Old 03-30-2011, 09:10 AM
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Quote:
Originally Posted by Philster View Post
Umm... this is getting interesting:

http://auto.howstuffworks.com/car-dr.../odometer1.htm

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.

While that does work on older mechanical odometers, it does not work on the new electronic ones


If anything, there are great photos of the mechanical inner workings.... along with the interesting conclusion...
I can absolutely guarantee that this isn't true for all mechanical odos. Newer versions had anti-reverse ratchets in them.
#19
Old 03-30-2011, 11:40 AM
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Ok, who's got a 1961 Ferrari 250 GT California in their garage to test? (Though to be fair, apparently the car in the movie was actually an MG in Ferrari clothing).

Last edited by fiddlesticks; 03-30-2011 at 11:42 AM.
#20
Old 03-30-2011, 12:52 PM
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Minor hijack question for Gary T, et al.

Many years ago I needed to change the rear axle (diff) ratio in my truck for towing. The dealer said they'd need to change (something?) in the transmission as well, in order for the speedo and odometer to measure correctly.

In a modern automobile, can this sort of change be done by software (by the dealer)? It seems like something that would be desirable, should the owner decide to change axle ratios, or tire sizes.

Just curious, with all the computers controlling cars nowadays.

Last edited by pullin; 03-30-2011 at 12:53 PM.
#21
Old 03-30-2011, 01:18 PM
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Actually, there are gears in most american cars to alter the spedometer and odometer performance for the use of different tires/wheel combinations. They still won't reverse. I had a 1960 Austin A40 that would back off the odometer while in reverse. Very shortly after that it became very common for it not to work.

Tris
#22
Old 03-30-2011, 01:50 PM
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As a slight tangent/hijack/rant, the newer mechanical odometers were also virtually tamper-proof. In addition to not rolling backwards, if you opened them up and messed with the numbers, it would disturb the clockwork on them and the numbers wouldn't line up any more. The only way you could change the reading was to actually turn the whole thing and, of course, it would only move forward so to roll one back you'd have to roll all the way forward, which would take quite a long time on a 6 digit odometer!

But then they started using the digital ones, in part because they were supposedly even more tamper-proof because they required exclusive dealer-only tools to change the reading on. Except that now there's cheap generic versions of these tools that anyone can buy off the internet, so rolling back an odometer is easier than ever!

I think the only reason why there hasn't been an epidemic of roll-back fraud is that mileage really doesn't affect the value of a vehicle as decisively as it once did, so there's less of a motive, and you're a lot more likely to get caught because of things like CarFax.
#23
Old 03-30-2011, 01:57 PM
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If you've been in a vehcle on ice, you notice that spinning the tires cause the speedometer to go up even if you're barely moving. I noticed this in a cab and wondered if frequently doing this upped the cabbies' take. SO the pickup is between the transmission and the drive tires. Probably not on one whel to allow for differential...

A friend many years ago bought a clunky old truck (late 60's); after fixing it up, we went on a road trip. He was amazed how smooth and quiet the ride was, until we stopped for gas 90 miles later and I calculated we had been doing 55mph not 70. I think the previous owner had among other things swapped the tires and rims for smaller ones.

Speedometer rollbacks have been a common complaint about used car dealers since the 60's. I assume the requirements to make spedometers tamper-proof and difficult to roll back date back to those days.
#24
Old 03-30-2011, 02:16 PM
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Quote:
Originally Posted by pullin View Post
Minor hijack question for Gary T, et al.

Many years ago I needed to change the rear axle (diff) ratio in my truck for towing. The dealer said they'd need to change (something?) in the transmission as well, in order for the speedo and odometer to measure correctly.

In a modern automobile, can this sort of change be done by software (by the dealer)? It seems like something that would be desirable, should the owner decide to change axle ratios, or tire sizes.

Just curious, with all the computers controlling cars nowadays.
Yes, there are several scan tools on the market that you can use to not only read your own OBD II engine trouble codes but to also adjust for changes in tire diameter, change the RPMs at which your automatic transmition will shift, and many other neat things.

Unless you are a car person it probably wouldn't pay to buy one of these tools for one time use. Take it to a shop and they will plug into your OBD II data port, usually located under the dashboard about where your gas pedal is, and set the new ratio. The OBD II data port looks a bit like the printer port on a computer, it is usually located on the driver's side of the car under the dash, because part of the sequence of doing the scan and clearing the codes, etc, involves turning the ignition on and off. Stick your head under the dash and look, it's there.

If the shop starts talking about expensive alterations and new speedo gears, go someplace else. This is something that the local tire shop should be able to do without any problem when you get bigger wheels.
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