Speedometer Theory

[tweiss3]

I am a bit confused about how our speedometer reads. After some playing around in the snow, I have observed a few things:

1) Speed apparently isn't read by transmission speed or the rear wheels, cause I can spin them or lock them and the speedometer won't read.
2) It doesn't read from the front wheels either, cause you can lock them while moving, slide, and the speedo still reads.

I had always thought speed was determined from a cable from the trans, but maybe not?

91 S5 NA. I don't have ABS, which is usually the other way cars read speed.

Can anyone enlighten me on the operation, I'm just curious and its bugging me.

On another note, my S5 handles like a charm in the snow, predictable, responsive, and hooks damn well, even when the temp is 10*F.

[josh18_2k]

uh.. clarify #1. because the speedo cable plugs into the trans.

[FC Zach]

And it is driven by a plastic worm gear.

"1) Speed apparently isn't read by transmission speed or the rear wheels, cause I can spin them or lock them and the speedometer won't read."

If there are moments that it wont read when you are rolling then you have a defective gear. They are plastic and its not uncommon for this to happen, I have replaced two. Easy replacement, just one 6mm bolt.

Edit: that or a defective and/or loose cable.

[tweiss3]

I say its not read by the transmission because while doing donuts (and spinning the rear wheels much faster than I was actually going) the speedometer isn't reading going any faster. Also, while going about 20mph through a parking lot, and pressing in the clutch and pulling the e-brake to lock the rear wheels, and effectively the transmission, the speedo still read my doing 20 at first and slowly decreasing to 0 when I had actually stopped.

That is what has me confused. I know my speedo is off by 5mph, but im not sure this is normal, or would be normal for a defective cable/worm gear

[RETed]

Your speedo is fucked.

I run my car on the dyno (both DynoJet and DynaPack), and my stock speedo has no problem showing speed even though the vehicle is sitting in one place.


-Ted

[josh18_2k]

or maybe you're only locking one wheel when you pull the ebrake.
with an open diff, the speedo could stay the sameish speed while sliding around. one wheel slow + one wheel fast, speedo will see the average of the two.

[Pete_89T2]

or maybe you're only locking one wheel when you pull the ebrake.
with an open diff, the speedo could stay the sameish speed while sliding around. one wheel slow + one wheel fast, speedo will see the average of the two.

Why would it average wheel speeds? As Ted described, the speedo is driven by a by a cable, which directly connects to a plastic gear in the transmission - it's spinning in direct proportion (based on gear ratio) to RPMs the transmission output shaft/driveshaft is spinning.

[t_g_farrell]

I say its not read by the transmission because while doing donuts (and spinning the rear wheels much faster than I was actually going) the speedometer isn't reading going any faster. Also, while going about 20mph through a parking lot, and pressing in the clutch and pulling the e-brake to lock the rear wheels, and effectively the transmission, the speedo still read my doing 20 at first and slowly decreasing to 0 when I had actually stopped.

That is what has me confused. I know my speedo is off by 5mph, but im not sure this is normal, or would be normal for a defective cable/worm gear

Unless your ebrake can lock up both wheels completely when you pull it your car
will still have at least one wheel turning and it will slow down gradually which
is exactly what your seeing with the speedo.

[tweiss3]

Doesnt the ebrake grab on both wheels. On ice i wouldnt think it were hard to lock both. As for speeilding up only one wheel, when im sliding sideways, I kinda assume im spinning both much faster than i am actually going. I guess this just means my cable is barely alive? I guess ill change it come spring time.

[josh18_2k]

whilst sliding with an open diff, only one tire is spinning (usually). power goes to the tire with the least amount of traction, which would be the spinning one.

when the tires are going different speeds, the driveshaft spins at an average of the two (divided by 4.1 obviously). if one wheel is going 10mph and the other is going 50mph, the speedo will read 30mph.

[tweiss3]

I completely understand what you are saying, however in 2nd gear at about 5k rpms, and the speedo still reads 10mph? I guess my real question was answered, my speedo and cable or something is screwed up.

[RETed]

I think you're just overcomplicating the whole thing by talking just about sliding...

Find a nice water puddle...
Brake stand the bitch.

What does your speedo do?


-Ted

[tweiss3]

Did that this morning on the ice patch at the bottom of the driveway, Speedo doesn't climb unless i really begin to move.

[tweiss3]

Well call me crazy, but now, since i began my commute this morning, its acting as you would expect, correctly. Wtf? Im not sure whats up, but apparently nothing is wrong anymore.

[My5ABaby]

Well call me crazy, but now, since i began my commute this morning, its acting as you would expect, correctly. Wtf? Im not sure whats up, but apparently nothing is wrong anymore.

Answer: You own a rotary. It does random ass things sometimes. Don't touch it if it's working right as it doesn't happen often. :)

[t_g_farrell]

Your cable may need to be lubricated. Or theres moisture in the speedo and its
affecting the magentic pickup for the spinning cable or something.

[josh18_2k]

usually when your speedo is on its way out the needle bounces. that gets annoying quick.. im going 60- no wait, 30, 80? wtf

when in doubt, go the same speed as everyone else lol

[tweiss3]

Lol, i use gps for my speed 80% of the time. Although, i do use cruise alot, and if tue cable dies im screwed in that mannor.

[NoDOHC]

I have the opposite problem, mine reads high when cold and makes this obnoxious squealing/grinding/whining sound (you almost have to hear it). After the interior warms up, it becomes quiet and semi-accurate.

ReTED, I am jealous that you have a functional speedometer on the dyno, mine apparently only reads up to about 80MPH (even though I was pulling to 165 MPH). The VSS to the ECU is accurate, but the needle on the gauge is not.

Obviously I wouldn't have any way of figuring this out on the street...

[RETed]

Guess I'm lucky?
But I did replace my original speedo cable soon after I got this car back in '97 or so...
I've maybe put on 30k miles on it since - it doesn't get driven that often.

It is kinda unnerving to put down a 4th gear run though as you see the speedo sweep past 100mph, and you're sitting inside going no where on the dyno runs... :eek:


-Ted

[tweiss3]

Hmm, I am assuming this is the original cable, at 140k now, and it consistently reads 5 higher than I am actually going. Consistent is enough for me. I am following the key rule, If it works at all, don't touch it.