I've got a kill switch in my car - mounted on the shifter. Saves wear on the ignition barrel, and makes it more convenient to use, since I'm usually going for neutral at the same time I want to kill the engine anyway.
FYI, some ideas on how-to...
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Kill switch how-to
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S10 Kill Switch
Have you got fuel economy instrumentation? That'll help more than anything else. It'll motivate you, show you what's working
and pay for itself in fuel savings.
Also, in case you haven't seen it:
- 100+ hypermiling / ecodriving tips to increase gas mileage - EcoModder.com