For the 1989 model year, a visible change inside was digital trip computer in the dashboard. At the same time Australian models received the same engine management setup as other markets. Porsche debuted the 928 GT in the late winter 1988/89 after dropping the slow-selling CS and SE. In terms of equipment, the GT was like the 928 SE, having more equipment than a Club Sport model but less than a 928 S4 to keep the weight down somewhat. It had the ZF 40% limited-slip differential as standard like the Club Sport and SE before it. Also, like the CS and SE, the GT was only available with a manual gearbox. European 1989 CS and GT wheels had an RDK tire pressure monitoring system as standard, which was also optional for the same year S4. For 1990 model year Porsche made RDK and a 0-100% variable ratio limited-slip called PSD (Porsche Sperr Differential) standard in both GT and S4 models for all markets. This system is much like the one from the flagship 959 and gives the vehicle even more grip on the track. In 1990, the S4 was no longer available with a manual transmission.