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?

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.

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.

So I guess the Ferris Bueller scene with the Ferarri was wrong.

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).

Nope. I was spot on accurate.

Their attempt failed to reverse the odometer.

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).

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.

In fact at one time it did work. Not sure how a mechanical one could be kept from reversing it.

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.

Engineers are clever.

Geez, spoiler alert!

Air-cooled VW beetles most certainly did run their speedometers off the front left wheel. May be true for other air-cooled VWs.

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.

Uh, no.

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.

Umm… this is getting interesting:

*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.

Ok, who’s got a 1961 Ferrari 250 GT California in their garage to test? :slight_smile: (Though to be fair, apparently the car in the movie was actually an MG in Ferrari clothing).

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.