Lois & Clark Forums Forums General Discussion Off Topic Car Mileage

Yes this is the most absolute epitome of me and my friend's boredom. How exactly can a car register that you've driven a mile? I did some research on the web (more evidence of my boredom LOL), but I didn't find much. So I figured I might as well ask you guys since there seems to be such a random bunch of knowledge floating around FOLCdom.

Jen

i'm not certain, but i've always assumed it was based on tire rotations. you know the circumference of a tire to a fairly good estimate.

let's say the tire has a 3 foot circumference (that won't be the actual number, but it's good enough for an example).

3 feet * number of times the tire has gone through a full rotation * 1 mile/5280 feet = miles the car has been driven.

you can do that digitally or you can set up a mechanical gear train to do the conversion for you.

not certain that's how it's actually done, but that's how i'd do it.

Paul

Paul posted just as I was going to post! He had I were thinking the exact same thing, though. I also always thought it was based on tire rotations with some sort of callibration factor. But I am not sure if I am right or not.

- Alicia

That's a pretty interesting theory. I like it...and if it's true (or remotely close), there would be some digital gadget somewhere in the car that keeps track of that sort of thing?

Jen

I know next to nothing about cars, let alone how their distance calculations work, but I do know my bicycle and it's components pretty intimately and I know that when I buy a new speedometer for it I have to input the circomference of my tire into the computer so that it can calculate the milage just like Paul explained. Wow, that was a long sentance.

~Anna

Very spiffy then. Thanks everyone, I shall report back to my friend about this...around 2am when my next wave of boredom hits *snort and grin*

Jen

I think the guys have it right - and I think that tire rotation also determines how fast the speedometer registers that you are travelling. I say that because when I was a young teenager, my mom drove my sister and I and some of our friends to summer camp one year in her new Honda Accord - well, it was used, but new to us Anyway, we got pulled over by a cop for speeding, after we had just been talking about the speed limit, and we had our eyes glued to the speedometer, which had been very steady on the posted speed limit. When the cop told my mom that she had been speeding, we all sort of gasped and said no way that couldn't be - and the cop let my mom off because it was so obvious how shocked we were. When we got home, Mom took the car to the mechanic, who discovered that the tires were just the slightest bit, um, smaller? I think... than the specs required, and that is why the speedometer was inaccurate.

Seems to me that the odometer is also connected to the tires, so if my mom had troubles with the speedometer, she might have also had troubles with the odometer too. Will have to ask her and see if she remembers...

Melisma (shutting up for a while, here under her Rock)

Yep, that's the answer. Wheel rotation; and it is important to buy the right tires for the car. The one that gets me though, is our car measures TIRE PRESSURE. That still amazes me, since there is no way it can actually measure the air pressure inside the tire. The best we can figure is that a sensor determines when the car is level. The car does not have to be moving for the measurement to be taken. The probability that all four tires will be evenly deflated is very small, therefore when one tire is low, the car tilts. It was right just last week when the tire pressure was down 10lbs in the right rear.
O.K. Now I'm even boring myself.

Artemis

Guys? I didn't know Anna and Alicia had sex change operations...or are they just cross dressers? Hmm.... <G>

Annie

Me too!
Artemis

LOL, I meant guys in the generic sense, not in the gender-specific sense. As in 'you guys'. Sorry - hope the girls weren't offended

Melisma (crawling under her Rock to feel like a heel - not the kind that's on your foot, BTW )

depends. most cars, it wouldn't be a digital gadget. some cars now have digial odometers, run by the car's onboard computer. those are gradually becoming more common.

still, most cars have an actual physical odometer. that would not be converted by a microchip. instead, it would be converted by mechanical means.

one way you could do that would be to use a gear train. let's say the circumference of the tires actually was 3 feet. in that case, 3/5280 = 1/1720. so, the dial that registers 1/10ths of a mile should turn (going up one number, not a full rotation) once every time the car's wheels turned 172 times. it should do a full rotation once every 1720 times the car's wheels turned.

now, the way a gear train works is this:

you take a big gear and a small gear and mesh them together. the small gear has to turn several times to get the big gear to turn once (number of teeth that the big gear has devided by number of teeth that the small gear has). otoh, if you put two gears on the same axle, they turn at the same rate. so, you put a small gear on one axle and connect it to a big gear on another axle. on the same axle as you have the big gear, you put another small gear. mesh that one with a big gear, and so on. you can set it up so that it takes exactly 1720 rotations of the first gear to turn the last gear once.

there are ways to do the same thing with an electronic signal that involve resistors and capacitors and such. that's most likely the way it's done in cars. you have a sensor reading the number of times the car's axle turns, then wires that go up to the odometer. those wires carry an electronic signal which would be processed mechanically, by running it through resistors and capacitors until you got the right signal strength. use those wires to activate a motor, and have that motor turn the dial on the odometer.

you can set up the odometer like one of those hand counter devices (you click it, and it counts the number of times you've clicked). the clicker turns the first dial, and the first dial is geared so that the one next to it turns once for every 10 rotatios of the first one. and so on down the line.

am i being clear? a bit behind on sleep right now, but i'm hoping i'm making sense.

yeah, i was wondering about those, too. the thought i had was this: put a strain gague on the inside of the wheel rim. run the signal up to an onboard computer. calibrate it with the properties of the rim (material properties, shape, etc). the strain on a given point on the rim will fluctuate due to tire rotation (i.e. there will be more strain when that point is near the bottom than when it's near the top), but if you keep track of something like the minimum or the average, you should be able to back out the pressure exerted by the tire. you won't get the most accurate reading, but it should be enough to tell you when the tire is going flat.

actually, you could set up multiple gagues. put four of the things on each wheel, spaced out around the rim, then average the input from each. that should help smooth out the variations.

of course, i'm not sure how tire changes and tire rotations and such would affect that. the strain gagues might become unreliable.

it's a thought, anyway. makes a lot more sense than trying to read the strain on the rubber with some kind of optical sensor.

i do like your level idea, but what happens if you're driving on an incline or something?

what i really want to understand is traction control. i guess you'd put a tachometer on each wheel, to see if one is spinning faster than the other? but then how do you correct for that? you'd need the left and right tires to be on seperate axles, wouldn't you? each with its own computer-controlled rapid-response gearbox or something. doesn't make much sense to me.

Paul

Ah ha! Here is where the Southern snob in me jumps up and down going "This is why we say 'y'all' not 'you guys!' Never refer to a lady as a 'guy.'"

See, for all of those out there who make fun of us for saying y'all, here's one good reason why we do. <G> Don't even get me started on people who look into a crowd and say "Do you ever..." You who? You her? You me? Oh, you're addressing *all* of us?" <G>

Annie (tireless defender of y'all)

ROTFL. I got such grief during high school because my dad's from New York, and I always said something that came out sounding like 'use guys'.
I finally broke down and started saying 'y'all' when I started college, but I just couldn't keep it up. Later guys.

Jen
Still the Tireless Defender of You Guys

I'll admit it, y'all really hurts my ears . I am from the North - I will NOT admit to being from the Midwest - Ohio is a Northern State East if the Mississippi - so I say "you guys" and I also say "pop" instead of "soda", but I would never call a girl a "guy!" So I was sort of offended being called a guy.

I might have said "they are right" but never "the guys are right" because I am not a guy and do not like to be referred to as a "guy" if that makes any sense.

I would use "you guys" saying "Do you guys want to go with me?" or something to that effect.

- Alicia

I spent a couple of years living in Alabama so I'm pretty familiar with the "ya'll" and then the rest of the dialect. (Ugh. I still get teased about the way i say "Down" I say something close to "Dayown" lol) However, I live near Chicago right now, so the "you guys" thing floats with me too. "You ya'll guys" try that one!

Caroline

For measuring tire pressure Paul said:
Yeah, that would work. But there is nothing connecting to the rims. We just had all 4 tires rotated. When we got in the car after it had been driven home from the tire place, the "low tire pressure" message came on. We were sitting in our driveway on an incline and had not moved the car. Off we went in search of air and a gauge. Yep, the right rear was 10 lbs low. It doesn't tell you which tire, but it sure was right.
All I can think of is the front axles not being aligned with each other or the rear axles not being aligned with each other. And all of that might come about because of the traction feature, which we have.



Artemis