celica gtr speedo fault, help please?

[BRIDDLES1]

i have a 1990 2L celica GTR import (st182), the car will be up for sale next week. £1750 ono
It has a 4 inch rear box, induction kit, hot wires leads, irridium plugs, powerboost valve, its got st202 alloys n 4 tyres like new.
I have put new clutch kit in, new timing belt and pulleys, new head gasket, bolts, and head set, new rear discs and pads, new front pads, and had the roof resprayed n re-lacquered. and 11 1/2 months MOT
The final thing to cure is a fault with the speedo....
The clocks are in mph already.....so it has possibly been converted b4 I got the car.....
You turn the ignition on and the speedo jumps to 50mph, when you drive the speedo goes up very quickly with your speed, so it reads 160 mph at approx 55 mph.
I have changed the speedo sensor but fault still there.
I have located a black box behind dash that plugs into a multiplug with six pins, and another multiplug with 3 green wires (also 6 pin type) next to it that fits back into the loom if you remove the black box.
If I unplug the black box with ignition on the speedo drops back down to 0.
could this box be delimiter and converter unit (says GDS, serial number 971081).
Cant find any boxs matching this on net but would rather plug another replacement in if this is faul than buy cheap converter and splice wires,,
Any help would be really really really appreciated as i just want to get money back on car.....