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Old 07-13-2011, 10:49 AM   #1 (permalink)
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Join Date: May 2011
Location: Hickory, NC
Posts: 32

Dodge Neon - '04 Dodge Neon SE
90 day: 39.86 mpg (US)
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Coasting in gear question

Sorry for the long post.

I do fairly well hypermiling, but I have a question about coasting. My 2004 Neon is a 5-spd and depending on the steepness of the hill I am descending, I will either take the transmission out of gear (if I don't need the engine breaking) or leave it in gear so that the fuel will cut off and increase my mileage.

My concern is that I was recently traveling from Asheville, NC to Hickory on I-40. Anyone who has traveled that road knows how long and steep I-40 gets on Old Fort. It is about a 4-5% grade for about 5 miles. I left my Neon in gear and the fuel cut out (I monitor this on my scangauge). For the first time I had the Water Temp on the scangauge because of the heat outside and I noticed something strange. The water temperature of the engine dropped to 196 after about 1/2 mile and stayed there (didn't go any lower).

Now I would think that if the fuel was shut off for the 5 minutes it takes to coast to the bottom that the engine would continue to cool the whole way to the bottom. I am traveling between 55-70 MPH, so air is flowing over the radiator (and the engine) the whole time.

I also would think that the thermostat is still open at this temperature because during the winter my car runs 190-192 and when coasting (engine on) I have seen as low as 185.

The reason I am wondering about this is when I checked my mileage for the tank coming from Asheville there was a BIG difference between what the scangauge read and what the actual calculated mileage was (~10%). I have to wonder if the engine is actually burning fuel but the scangauge doesn't see it.

Any thoughts?

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