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Old 01-22-2021, 09:28 PM   #22 (permalink)
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Join Date: Dec 2011
Location: New Zealand
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ND Miata - '15 Mazda MX-5 Special Package
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I looked up the stock thermostat for your engine and it's a 190F part - that is, it starts to open around 190F. I would not expect it to be fully open until around 205F, and the fan likely doesn't come on until at least 5F higher than that. I'd wager ideal operating temperature is ~200-210F, where the thermostat is fully open but the fan hasn't come on yet.

I block my grille completely in winter. On most days, my car still isn't fully warmed up by the time I get to work, but the grille block has a huge positive impact on economy. Occasionally my radiator fan comes on, on longer drives. While this wastes energy, the economy loss of a slower warmup outweighs the electrical cost of running the fan now and then.

I'd recommend a grille block, and to play around with the size until you find the point where your radiator fan isn't coming on *most* of the time under normal driving conditions. Attach it with something like zip ties, and keep a something in the car to remove it (scissors or w/e) in case you need to.

Edit:a kill switch is likely also low hanging fruit. The less efficient an engine is, the larger the gain from coasting without it running. I generally only use my kill switch when coasting down a long hill, but even ~60 seconds of coasting in a 20 minute drive can save a lot of fuel.
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