what does your m20b25 get mileage wise?

[nando]

there's a potentiometer on the back of the speedo's circuit board.

I don't think you can adjust the calibration of the OBC and odometer quite as easily. the speedo is set to read high in purpose, but the OBC and odometer are meant to read correctly.

[dogsbark26]

What? We just discussed that all three are derived from the same signal. Surely that adjustment (potentiometer) effects the signal before any of the other systems use it? No?

Even so, the car has been operating with slightly higher mileage reading for decades (right? the odometer mileage would read higher because it thinks I am going 64 when the car is actually traveling at 60).

Anyway, I am genuinely unconcerned if the mileage is somewhat overstated. It would be nice though if I could look at the speedometer and know how fast I am going without having to "adjust" mentally for known offset.

[Earendil]

What? We just discussed that all three are derived from the same signal. Surely that adjustment (potentiometer) effects the signal before any of the other systems use it? No?

Even so, the car has been operating with slightly higher mileage reading for decades (right? the odometer mileage would read higher because it thinks I am going 64 when the car is actually traveling at 60).

Anyway, I am genuinely unconcerned if the mileage is somewhat overstated. It would be nice though if I could look at the speedometer and know how fast I am going without having to "adjust" mentally for known offset.

Without actually knowing the specifics, I can make an educated guess based on the electronics class I hated taking in college.

It it uses a potentiometer to adjust, it is probably adjusting the voltage to the motor that drives the needle, and not to the signal. The signal from the sensor is a pulse, and would not be effected (in a good way) by a potentiometer.

[Earendil]

Anyway, I am genuinely unconcerned if the mileage is somewhat overstated. It would be nice though if I could look at the speedometer and know how fast I am going without having to "adjust" mentally for known offset.

There are better things to be concerned about :)
It is generally accepted that all BMWs read 2-3 mph faster than you're real speed. If you start using a slightly larger than stock diameter tire (snow tires often are) it would read correctly, instead of reading slower than you are actually traveling. I've always assumed it was configured this way so that as long as you ran a tire that was "reasonable", that it would never tell you that you were going slower than you really are.

That said, I do run tires on my E30 that are slightly taller, and my speedometer is SPOT on. However, I switched gauge clusters a little while back, and to my surprise and confusion, BMW slapped a different gauge FACE on there at some point, that was rotated 5mph off. The needle reads exactly the same angle wise, only the location of the numbers of the face are off. So my car currently tells me that I am going 5mph faster than I really am (reads 30mph, am going 25mph). So far no one that borrows my car has ever gotten a ticket :up:

The OBC remains as accurate as ever.

[stamar]

There are better things to be concerned about :)
It is generally accepted that all BMWs read 2-3 mph faster than you're real speed. If you start using a slightly larger than stock diameter tire (snow tires often are) it would read correctly, instead of reading slower than you are actually traveling. I've always assumed it was configured this way so that as long as you ran a tire that was "reasonable", that it would never tell you that you were going slower than you really are.

That said, I do run tires on my E30 that are slightly taller, and my speedometer is SPOT on. However, I switched gauge clusters a little while back, and to my surprise and confusion, BMW slapped a different gauge FACE on there at some point, that was rotated 5mph off. The needle reads exactly the same angle wise, only the location of the numbers of the face are off. So my car currently tells me that I am going 5mph faster than I really am (reads 30mph, am going 25mph). So far no one that borrows my car has ever gotten a ticket :up:

The OBC remains as accurate as ever.

went ahead and did a search at discount tire for 195 70 r14 and 205 65 r14

there are no h rated tires in those sizes.

Do you run passenger tires?

According to tire manufactures tires that high are not made for speed.

[nando]

you aren't going to find much in 14" these days.

[nando]

What? We just discussed that all three are derived from the same signal. Surely that adjustment (potentiometer) effects the signal before any of the other systems use it? No?

Even so, the car has been operating with slightly higher mileage reading for decades (right? the odometer mileage would read higher because it thinks I am going 64 when the car is actually traveling at 60).

Anyway, I am genuinely unconcerned if the mileage is somewhat overstated. It would be nice though if I could look at the speedometer and know how fast I am going without having to "adjust" mentally for known offset.

You need to understand how the speedo works before making assumptions. It has three outputs, one is a digital output that goes to the OBC and DME. One is a motor output that goes to the actual motor behind the needle. the other goes to the motor that drives the odometer. the digital and odometer outputs are accurate. the speedo needle is purposely calibrated to read high.

the speedo needle can be calibrated with one of the resistors connected to the chip that processes all the signals. I haven't dug too far into the other signals but I suspect they can be calibrated as well.

[nando]

Without actually knowing the specifics, I can make an educated guess based on the electronics class I hated taking in college.

It it uses a potentiometer to adjust, it is probably adjusting the voltage to the motor that drives the needle, and not to the signal. The signal from the sensor is a pulse, and would not be effected (in a good way) by a potentiometer.

http://www.datasheetcatalog.org/datasheets/700/489248_DS.pdf

[dogsbark26]

Truly fascinating. Thank you all for sharing.

[Earendil]

went ahead and did a search at discount tire for 195 70 r14 and 205 65 r14

there are no h rated tires in those sizes.

Do you run passenger tires?

According to tire manufactures tires that high are not made for speed.

Basically I just need to stfu and not post math/logic on Mondays.

What I said about taller tires is correct, but what I said about my tires is not. I actually run a smaller diameter tire in the summer/track, in a 205/50/15. This ends up up moving my speed further from the mark by 3.8% according to the all too famous tire calculator page.

You need to understand how the speedo works before making assumptions. It has three outputs, one is a digital output that goes to the OBC and DME. One is a motor output that goes to the actual motor behind the needle.

AFAIK, you are the first one on the internet to say what the output is, let alone that it's digital. I spent days a few month back trying to figure it out, as I was trying to build an Arduino that would read it and provide calculations to an iPhone. It was an awesome project until the "output speed" pin on the back of the cluster produced nothing my Arduino was able to pick up. Without a good oscilloscope I didn't have an easy path forward to trouble shoot. I hadn't guessed that the signal would actually be digital though. I assumed it would just pass along the raw input.

The data sheet you provided, is it for the chip that the VDO gauge cluster uses, or just as an example?

This thread is officially derailed.

[nando]

all you gotta do is take it apart and look. :)

as it turns out - most of the pins on the green connector behind the speedo don't go to anything. really. I think only 2 are hooked to an actual VSS. one goes to a 12v signal. a couple more connect to nothing at all.

that's the VDO chip. I think the Motometer cluster uses the same or a similar one. it was used in almost every euro car from the time period. all I did was look at the chip and google search the markings.

[Kozworth]

II appreciate the clear up on the obc, i have one quick question. It seems the backlight is out, its hard to read dring the day and forget about it at night. Does this obc light up brighter or do i have to switch it/replace an led

[nando]

they're burned out. replace with some red LEDs, all you need is a 330ohm resistor.

[iwantspeed]

At yesterday's autocross
:)

[stamar]

Basically I just need to stfu and not post math/logic on Mondays.

What I said about taller tires is correct, but what I said about my tires is not. I actually run a smaller diameter tire in the summer/track, in a 205/50/15. This ends up up moving my speed further from the mark by 3.8% according to the all too famous tire calculator page.




AFAIK, you are the first one on the internet to say what the output is, let alone that it's digital. I spent days a few month back trying to figure it out, as I was trying to build an Arduino that would read it and provide calculations to an iPhone. It was an awesome project until the "output speed" pin on the back of the cluster produced nothing my Arduino was able to pick up. Without a good oscilloscope I didn't have an easy path forward to trouble shoot. I hadn't guessed that the signal would actually be digital though. I assumed it would just pass along the raw input.

The data sheet you provided, is it for the chip that the VDO gauge cluster uses, or just as an example?

This thread is officially derailed.

sorry if im just hearing what I want to hear. I want to get tires that make the accellerator accurate. seeing as 14 inch ones dont exist,

I found them in 15 inch sizes http://www.discounttiredirect.com/di...05&ar=65&rd=15

a couple brands ive never heard before and the standard falken 512 and 912

and in 16 inch wheels

http://www.discounttiredirect.com/direct/searchTiresBySize.do?sw=false&cs=205&ar=60&rd=16

next I have to ask, does this rub? I have never heard of too many e30 owners going for this size it would decrease 0-60 time but it would increase gas mileage....

but strangely enough, my car once had wheels and tires on it this circumference. the seller switched to bottle caps when i got it.

edit riken is actually a japanese brand owned by michelin. looks really good nothing but good reviews.