Sounds like someone has converted it to mph but not delimited it.
Standard limiter kicks in at 180 kmh = 111.87mph
Miles per hour (mph) ---> Kilometers per hour (kmh): mph x 1.609 = kmh
Kilometers per hour (kmh) ---> Miles per hour (mph): kmh / 1.609 = mph
You need to get hold of a delimeter & convertor kit. Most of them are a little black box that goes behind the speedo and connects up to a few wires on the back of the speedo. They basically intercept the signal from the speed sensor, divide it by 1.6 and send it on to the ECU so you don't get the speed limiter cutting in.
You'll probably find a black box already spliced in place behind the speedo (although some early ones were spliced in down by the ecu I believe). You'll need to remove this original one and replace it.
On the other hand it could be a delimter / convertor that has been wired up incorrectly - passing a signal / 1.6 to the speedo but an original signal to the ECU.
Have a look at
http://www.gtiroc.com/testgtirocspeedo.htm
for more info.
Cheers
GeeTee 8)