Speedometer

I have a 1980 Buick Regal. I'm having problems with the speedometer it seems to be off by 10mph sometimes ahead sometimes behind.

It's a custom regal I put a 350 chevy in it with a 200-R4 transmission. I'm not sure what kind of specs you might need to help me with this problem but any information on how to solve this will be much appreciated. Thanks.

Your car has a mechanical speedo, which is hooked to the trans via cables. If it still has the cruise hooked up, there will be one cable from the speedo to the cruise, then one from the cruise to the trans. The lubricant in the cable housing can get thrown out, causing the cables to bind and cause the speedo to jump around. The ends of the cables are square, and the pieces that drive them have square holes and are made of plastic. As they age, they can strip out and also cause irratic speedo operation.

Your car has a mechanical speedo, which is hooked to the trans via cables. If it still has the cruise hooked up, there will be one cable from the speedo to the cruise, then one from the cruise to the trans. The lubricant in the cable housing can get thrown out, causing the cables to bind and cause the speedo to jump around. The ends of the cables are square, and the pieces that drive them have square holes and are made of plastic. As they age, they can strip out and also cause irratic speedo operation.

I know this can't be the problem as the cable is only about a year old, bought it new.

It could be the cruise box, or the adaptor at the trans that the cable screws into. Is the trans the original one, or did it come with the engine?

It could be the cruise box, or the adaptor at the trans that the cable screws into. Is the trans the original one, or did it come with the engine?

No isn't original. The original to my car was a 3.8L V6 w/TH-350. So I knew it would cause speedometer problems but I'm not sure how to solve it. The transmission came off a Monte Carlo.

The probelm is most likely the gear/adaptor in the tailshaft isn't correct for your car's loadout. The Monte's had both the retangular long sweep speedo, and a round one (if it had a tach). All but turbo Regals came with the retangular sweep speedo's, which are calibrated differently than the round gauged clusters. Last but not least, never eliminate a part just because you replaced it.

The probelm is most likely the gear/adaptor in the tailshaft isn't correct for your car's loadout. The Monte's had both the retangular long sweep speedo, and a round one (if it had a tach). All but turbo Regals came with the retangular sweep speedo's, which are calibrated differently than the round gauged clusters. Last but not least, never eliminate a part just because you replaced it.
Yeah I have thought about this. However with the 200-4R there is no way to change that out with the correct one as the transmission solid without a removeable tailshaft. And yeah my Regal has the rectangular speedometer.

Well after a bit of research it seems as I am able to change out the spedometer gear it's just tricky. I think this may be my problem so I'll look into this some more and as you said I won't eliminate a part just because I replaced it.

The speedo drive gears are color coded for calibration purposes (different gear ratios for example). If you take it to a reputable trans shop, they should be able to tell you what color speedo drive gear you need for your rear gear.

The speedo drive gears are color coded for calibration purposes (different gear ratios for example). If you take it to a reputable trans shop, they should be able to tell you what color speedo drive gear you need for your rear gear.
Yeah I found a site that will calculate which driven gear color I need. If I can't figure it out I'll take it to the place that re-built it for me. Thanks for your help richtazz it's much appreciated.