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Old 12-25-2007, 06:16 AM   #5 (permalink)
EcoModding Apprentice
Join Date: Dec 2007
Location: Melbourne
Posts: 152

The Mighty Mira - '92 Daihatsu Mira
90 day: 61.32 mpg (US)
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Thanked 7 Times in 7 Posts
Originally Posted by Lazarus View Post
Very nice job.
Thanks! To you, and everyone else who commented.

Can't wait to see some FE numbers.
Me too. Won't be happening for a couple weeks (time off for Christmas), so no commuting. As it is, it takes 6+ business days to empty a tank, where it used to take 4 or less.
What do you think you will end up with about 3% increase?
Good question.

Let's talk about drag coefficient first. If we use the figures from the original Volvo paper, which basically formed the impetus for DIY aero FE modifications in the last couple years, drag contributed by the front wheels is 13.1%. Of what? Since the best case drag coefficient for close to the ground cars is around 0.11, and the drag contributed by the "exterior" is 31.7%, I'm estimating that the car they started out with had a Cd of 0.35. It seems a fair assumption, giving that cars are often around that - mine is 0.37.

13.1% of 0.35 is 0.0455.

Whether we can eliminate all that is questionable. I'm going to shoot for 10%, or 0.035.

If I did the coast down test properly, my drag coefficient was 0.26 (rounded up). My new Cd should be somewhere in the area of 0.26 - 0.035 = 0.225.

That's a 14% improvement, significant to my eyes.

On the highway (i.e. where I don't have to stop), I will see almost all of that gain, because I pulse and glide (proper glide most of the time) and air resistance is the dominant form of load. Where there are unavoidable stops, I will see less of that gain, because drag coefficient does not help with storing kinetic energy that is shunted into brake pads.

I'm estimating on my commute I will be 5% better off for having done this.
"Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed." - Isaac Newton
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