It sounds like you have the connections for the speedometer correct. Didn't t- you change your transmission from an auto to a manual? If so you need to change the speedo drive gear in the transmission as the auto cars and manual cars have different final gears and would cause your speedo error.
You can do this on the car. The area where the speedo cable goes in the transmsission is removable, there is a bolt next to where the cable goes in, remove it and the drive gear section will come out. You need to take the nylon gear from your automatic and put it in your manual transmission. This will fix your speedo error, assuming you gauge works correctly.
If you get a chance, check out the yahoo group for Volvo 1800s, there are lots of smart guys over there.
Modified by Stamperman at 7:03 PM 5-12-2009