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Diesels aren't often discussed here, but I would think they would respond well to eliminating intake restrictions (adding a cold air intake, etc). The trade-off here will be at least noise. The aerodynamic mods discussed elsewhere and by aerohead will help most on the highway, and Mullet's weight reduction suggestions will help especially in the city. Keep the speed down, keep up on maintenance, maybe a low-viscosity synthetic oil. Not much you can do for a diesel.