Hi.. I recently purchased a 1990 C2 Cab. Tiptronic. I love the car, however, it "bugs" me that the on-board computer reads a higher speed than the speedometer does. Upon checking it out, at 50MPH the on-board reads 3 MPH higher than it should; and at 70MPH it reads 5 MPH higher. The speedometer is the more accurate reading. My question is: Is there any way of adjusting or calibrating the On-board Computer to match the speedometer and show accurate speed?

I assume all the trip functions are off also.
Thanks for any help.
Rich

 

Dear Rich,
It is possible to calibrate the OBC but the cost is high (labour intensive) and finding a suitably experienced facility is also hard.
It is something that you have to put up with. You could try and find a 1992 model year tip OBC. These are generally more accurate. If it really bugs you maybe you could try and borrow one and find out. You should be aware that OBC speed is not legally required to be accurate as compared to the speedo and therefore the calibration quality is not as good.
Ciao,
Adrian
964C4

 

Funny- my BMW is the opposite (as are all 5-series per posts on a popular BMW forum). If I go 90MPH and go to average speed, hit reset and hold the speedo at constant 90, its something like 84mph really. I agree- it bugs the crap out of me that the speedo is off- I always have to adjust it in my head...the few times I hit 140MPH, I was honest and told the passengers we actually hit closer to 130 or so

I believe my porsche speedo is optimistic as well (and I think my '87 and '78 911 speedos were as well)...so much for spending the money on VDO guages if they aren't even accurate...

 

oops I sent the same message twice- disregard this post...

 

Rich,

I had the same problem with my turbo 3,6 - to much speed discrepancy between the speedo and the OBD speed display. After I replaced my speedometer with a (200mph) 993 turbo item (vs 180mph on my OEM), the two speed indications now go side by side.

Whether they are both right or both wrong now I do not know, however it is a lot less confusing as I don't have to chose/disregard any one of the two, and believe it or not they go side-by-side across the whole speed range I have driven - from standstill to an indicated 183 MPH - Coincidence or pure luck -I do not know but- definitely one less thing to worry about!

 

Hi.. Thanks Adrian and others. I am still interested in getting this fixed or trying a new unit. I am very capable of my own mechanical work and will continue to gather information to accomplish this repair. I will report back with my results. In the meantime, I am hoping for more information here about the OBC and its operation.
Rich

 

Dear Rich,

How can you be sure that your analog speedo is more accurate than the digital readout? Did you check it with a laser gun or any speed measurement device?
I had the same problem and asked this similar question before ( about 10 months ago ) and in fact I checked the distance and fuel consumption etc very carefully and found that the OBC gave me accurate readings. Based on that I tend to believe that the analog speedo is optimistic like Andrew911 suggested.
Adrian can help me in this, if it is an electromagnetic pick up like a tacho then it needs good calibration. Otherwise an optical/digital pick up ( encoder) should be more accurate and less calibration problem. I do not know what speed sensor my 964 is using( may be both ??)
I know Adrain told me once before and would like to clarify it again. Please Adrian!

Eddie

 

my is opposite, my speedo is about 10 mph faster than digital read out. I just beleive the faster one. but would be great if they can match.

tenn

 

Eddie,
After reading your post, I thought I would check again. It turned out that I was somewhat wrong. Covering an exact 3 mile flat road stretch using a stopwatch, at indicated 60MPH, I found that I was actually traveling 58
MPH using the OBC, and 61.4 MPH using the speedometer. There is over a three-MPH difference, however, neither was really 60 MPH.

Now, which one is easier to adjust to the other? I can live with the inaccuracy if they both read somewhat the same.

Rich
P.S. = Actually, I CAN live with it the way it is.

 

Dear Eddie,
You have to remember that you are allowed tolerances in these systems. The Tip uses a transmission mounted sensor to the transmission control unit then to the signal frequency converter (R52) then to the speedo and OBC which have their own tolerances. 1.4 - 2 mph is around 3% maximum error. Personally I do not think you will be able to adjust this out. You will need some seriously stable input signals to improve the instrument adjustment but the signal sources cannot be calibrated. The signal from the transmission sensor cannot be calibrated.
Ciao,
Adrian
964C4

 

Dear Adrian and Rich,

Thanks and totally agree that around 3% tolerance is acceptable. I tend to think that it seems to me that the OBC reading is lower than the analog speedo reading. I was told that it is quite normal that car manufacturers normally amke their speedo slightly optimistic with some obvious reasons. And as long as it is within the legally allowed limit.
Hey Rich, do you mind telling me the tyre size and inflation pressures on your tyre? Interesting experiment but really the average value mathematically cannot assure your accuracy of the meter at a particular instant of time.
I used to work for a British( been German and now back to British) Car manufacturer and they have a very tight and accurate speed measurement system but really 3 to 5 % is allowed and you could frequently find from the Car magazines that the top speed and the accleration time are different to the claimed figures from the manufacturer. If I remember corerectly, most of them long in the speedo readings.
As I said before, I am impressed with the OBC and tend to trust the OBC reading more.
Interesting finding anyway.
Cheers!

Eddie
91 C-2

 

To Eddie,
My tires are 255/40-17 Rear, 205/50-17 Front, S02 Bridgestones at 36 lbs.

Almost new.
Rich

 

Dear Rich,

Thanks, the reason I asked is that sometimes people change to a smaller diameter and does affect the speedo reading. Interesting to see you have 36 in the rear, I put 44 in the rear initially Like the manual suggested but found that it was too bouncing so I put them on 40 and I tried 36 but may be a shade too low for me. So how do you find the handling like using 36 all round?
Cheers!

Eddie
91 C-2

 

Eddie, I believe 36 PSI all around is the factory suggestion for 17" tires. 44 PSI for rear is for 16".

 

Sorry! You are right, my fault.
I do find 36 a shade too low in the back.
Really I should be more careful next time in writing here!
Sorry!

Eddie