I replace mine when they leak or when I start feeling them not working well anymore. I've noticed in several cars that the right front will fail completely while the left front is still working fairly well. I replace them in pairs.

As for the question on tire pressure, the ideal pressure for a given application will vary depending on speed, load, and the driver's priorities. For max fuel economy, run max pressure. For best ride quality, use the lowest pressure that still provides acceptable handling and doesn't overheat the tire (TRA yearbook is helpful for figuring that last bit out). For best handling, it will vary somewhat with track conditions and the only way to get it dialed in really well is with trial and error. At present I'm running around 37 all around but getting ready to start messing with stagered tire pressures to see if I can tweak the handling to my liking a bit more.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.