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 the 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. The S4 and GT variants halted production at the end of the 1991 model year, making way for the final version of the 928. The 928 GTS was available for sale in late 1991 as a 1992 model in Europe and in spring of 1992 as an early 1993 model in North America. Changed bodywork, larger front brakes and a new, more powerful 5.4 L, 350 PS (257 kW/345 hp) engine were the big advertised changes; what Porsche wasn't advertising was the price. Loaded GTS models could eclipse US$100,000 in 1995, making them among the most expensive cars on the road at the time. This severely hampered sales despite the model's high competency and long standard equipment list. Porsche discontinued the GTS model that year after shipping only 77 of them to the United States. Total worldwide production for all years was a little over 61,000 cars. Second-hand models' value decreased as a result of high maintenance costs due to spare parts that are expensive to manufacture. Many parts suppliers and enthusiast networks exist, especially in the United States, Germany and the UK. Road & Track magazine published a speculative piece in their April 2006 issue regarding the possibility of a new, 928-esque coupé that may debut on a shortened version of the Panamera's platform sometime around 2011 or 2012 model year