1990 Trans Sport Speedometer reading high

As in a previos post, my speedometer reads too high. Four or five years ago, it was about 6mph high when it read 70mph. Now it is about 10mph high. Since it continues to get worse, recalibrating the speedometer doesn't seem to be the answer. I'm thinking about replacing the instrument cluster. The tach doesn't work reliably either. In the meantime, we use a gps when on trips to assist in determining actual speed. However, I'm wondering if I could hook something up to the OBDI connector. It seems that one of the pins gives a 2000 (or is it 4000?) pulses per mile output. I'm thinking there ought to be a way to translate that into mph. Any suggestions? Any circuit diagrams?

May be this can help
www.avigex.ca/xport/speedoerror.jpg

www.avigex.ca/xport/instrumentpanel.jpg
www.avigex.ca/xport/speedsensor31.jpg

THere is no indication the OBD1 connector receives the signal; there are very few active pins in the OBD port. Signal would have to be taken at the ECM or the cruise control module or speed sensor buffer.

Speed sensor buffer is located behind instrument panel compartment above convenience center.
www.avigex.ca/xport/convenience_center.jpg

Yes. I don't know what I was thinking of. I guess that I looked at the speedsensor diagram and got lost. I could take the data off the speedsensor, but then I'd still have to build a circuit to transform it into speed and I'm not a digital circuit designer.

After reading your reply, I went back to Google and searched for ALDL Serial Data and found what I wanted.

It turns out that the ALDL serial data stream (Pin E in my 1990 Trans Sport) contains the speed, and RPM, information. (Maybe. It is difficult to be certain. I would have liked to think that GM used a standard format data stream for all its vehicles, but I know that's not true.) It is not easy, but there are ways to interface that connector to a PC/laptop and read the data. If I ever get around to working that out, perhaps I'll see if I can get a readout from a simpler device than a lap top. (Of course, I could break down and spend $700 to $1000 for the tool that does it, but I'd be better off buying a new vehicle.)

Thanks for your reply.

By the way, I do believe in miracles. They happen in my life every day.

.......may be in life, not in cars......