Take a look at your old clocks with the clear plastic cover off them.

Put your finger under the speedo needle where it sits at zero, move the needle upto about 60 mph and then let it go, it should drop straight down and maybe bounce a little and then settle in 1 spot, make a note of exactly where it settles.

Now take the plastic cover off your new clocks and repeat the same steps and see how it stops, you may not have put the needle on in the right place and this should determine how far out you are, take the needle off and put it backon again until it does exactly the same as the old clocks.

This should sort it for you, pop the clocks back in (it doesnt matter if the cable comes out of the end of the holder cos when the holder gets clipped back in place it will be ok) and take the car out for a spin with a mate in a car beside you, now check that your speedo reads the same ( give or take a bit) as your mate's

hope this helps

cheers

Rich