Tell them you want a Terratrip japanese gearbox probe. This is a sender for rally tripmeters. It's about 20mm deep, and is sandwiched between your gearbox and the speedo cable. No need to cut the cable, etc. You simply unscrew the speedo cable from the gearbox, screw the Terratrip probe onto the gearbox, then screw the speedo cable onto the Terratrip probe.