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For my 1990 coupe automatic, ever since buying it as a project and installing a rebuilt engine into it it has driven with a speedometer discrepancy. The offset is linear, as in always off by a certain percentage - for example at 30mph actual it reads 32mph, and at 60 actual it reads 64mph. These are rough measurements (using a GPS app to measure speed while cruising) but speedo seems to be consistently around 6-7% high.
Wheels are stock and tires are stock size, and as far as I can tell other than the engine the car is 100% stock. Unfortunately the RPO codes were lost - the sticker was gone when I bought the car so I don't know what axle ratio it has.
Could there be a mismatch between the rear diff ratio and what the computer was configured for? Like I said I don't see evidence that anything was modified from original, but I guess it's possible. Where is the configuration different between cars such that the speed measurement corresponds to the ratio installed in a particular car? Is it a different sensor at the transmission, or a difference in the gauge cluster programming, or ?
Or am I barking up the wrong tree and it's something else entirely?
Also is there an easy way to tell which diff ratio I have (without RPO codes)?
For my 1990 coupe automatic, ever since buying it as a project and installing a rebuilt engine into it it has driven with a speedometer discrepancy. The offset is linear, as in always off by a certain percentage - for example at 30mph actual it reads 32mph, and at 60 actual it reads 64mph. These are rough measurements (using a GPS app to measure speed while cruising) but speedo seems to be consistently around 6-7% high.
Wheels are stock and tires are stock size, and as far as I can tell other than the engine the car is 100% stock. Unfortunately the RPO codes were lost - the sticker was gone when I bought the car so I don't know what axle ratio it has.
Could there be a mismatch between the rear diff ratio and what the computer was configured for? Like I said I don't see evidence that anything was modified from original, but I guess it's possible. Where is the configuration different between cars such that the speed measurement corresponds to the ratio installed in a particular car? Is it a different sensor at the transmission, or a difference in the gauge cluster programming, or ?
Or am I barking up the wrong tree and it's something else entirely?
Also is there an easy way to tell which diff ratio I have (without RPO codes)?
You should be able to take your VIN to a GM dealer (parts dept) and they can confirm RPOs that it was built with. Just ask them to print you a SPID label of options for the car. For cars older than '90 that won't work but for '90 and newer it will. If they tell you NOT, it's possible it's a younger fellow that just doesn't know.
The fact that the variance is consistent that's a very good sign that a driven gear should correct it.
You will then need to confirm what's in it currently. You could actually turn a rear wheel 1 rotation and count the turns of the pinion flange to calculate current ratio also.
Your tires are 275/40-17?
*** A very sharp parts guy should be able to tell you which driven gear should have been there when built also.
) but speedo seems to be consistently around 6-7% high.
Because the error is consistent, changing the speedometer sensor's driven gear tooth count (located on the driver side rear of the transmission) will recalibrate the reading. You don't need to know the axle ratio to change the calibration because you know what the percentage of error is across the range.
Basically, you need to add 3 teeth to what is in there now. If it has 42 teeth you need 45 teeth. 42 / 45 = 93.33 or a 6.66% reduction in reading. (That's pretty close to "6 to 7% high")
There are different housings for the different tooth counts.
Pull the housing and report back with the color of the gear and tooth count. Also the color of the center and connector housing of the VSS itself. (That tells if the existing sensor can work with the required tooth count.)
Great explanation - thank you. Will pull it and check in the coming days.
Early VSS is easily busted removing. I believe the easiest (most effective) is find out what it was expected to have been built with first. Confirm in some fashion what is there and proceed.
A '90 w/2.59 ratio would have a 36 tooth 'white' and a VSS w/34-39 embossed on it, a 2.73 would be 38 tooth 'blue' w/34-39 on the VSS, a 3.07 would have 42 tooth 'green' w/40-45 embossed on the VSS. I believe 34 - 39 VSS is white and 40 - 45 is black if it's an original center-connect VSS. Any of the correct 'driven' are difficult finds these days, the correct VSS to match the gear needed are also difficult finds. All A4 '90 use the same 'drive' gear.
I'm not disputing IHBD math but you can NOT rely on the adding of 3 teeth to what you find.
Besides getting the RPO/SPID information from the dealer will help you down the road.
A visual of the VSS in the car without removing would be a very good first to do. WHITE 34 - 39 OR BLACK 40 -45. Take nothing apart!!!!!!
Speedometer offset mystery
