Quote:
Originally Posted by 3-Wheeler
It is this author’s opinion that the conditions and/or measurement methods during these coast down tests are still too variable to be of much use regarding aero improvements to our cars.
Jim
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Jim,
I've been studying your data and I have one suggestion as to a possible error source.
We all know how very sensitive the Insight fuel economy measurement is to tire pressure. Looking over the part of the year you were testing, I'm wondering if you may have had significant tire pressure variations. Tire temperature changes tire pressure. On sunny warmer days, the road would have been warmer, and on cold overcast days the road surface would have been colder. Cold stiff tires have higher rolling resistance.
I have a slight hill in front of my house which runs for .4 miles. When I give my Insight a little push, I gain speed to 8 MPH at the first intersection in the summer. In the winter, I can barely make 5 MPH at that point. You say that the car is thoroughly warmed up, but perhaps the tire temperatures are still varying significantly on different days.
From my racing I also know this. We use a bit more pressure on cold days to put the traction into the sweet spot.