Anyone about to order an executive saloon without an optional satellite navigation system should think again. Not having this desirable option could seriously impair its resale value and potential. Fitting sat-nav to luxury cars has been essential for decent resale for several years now but it’s fast becoming as much of a must-have option as metallic paint on smaller saloons. The BMW 5-series (right) is just one of the cars where ordering sat-nav could actually make you money come resale time. Add the cheapest £1185 guidance option to a new 530d, for instance, and the trade price guides indicate that it will add £1500 to its trade-in value. This sort of gulf is reflected on the forecourts, too. We went shopping for a nearly-new version and found two near-identical 530ds on the same forecourt. They were both autos, with leather, silver metallic paint and 9k on the clock, but the sat-nav car was going for £31,995, a grand more than the other model. Likewise two near-identical 53-plate Mercedes E320 CDi saloons that we spotted differed in price by £1200 depending on whether it was fitted with the COMS pack. Away from exec saloons, it pays to order sat-nav on other high-image models. Specifying the sat-nav option on the Mazda RX-8 should make you more when you sell it on. Although it’s still early days for used examples of the £22,000 four-door coupé to be on the forecourts, you’ll get all your money back if you’ve spent £1500 on the sat-nav. And salesmen are reporting that a sat-nav equipped model will fetch the same £1500 more in part-exchange, too.