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Old 11-27-2007, 10:33 PM   #10 (permalink)
MetroMPG
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Location: 1000 Islands, Ontario, Canada
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Blackfly - '98 Geo Metro
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09-10-2006, 08:11 Pm

Recorded a couple of runs this evening with the undertray on.

This really didn't help much, as you'll see in a minute.

Conditions:

Observed at: Grenadier Island 10 September 2006 7:00 PM EDT
Temperature 14° C / 57 F
Pressure/ Tendency 102.7 kPa↓
Humidity 61 %
WindNE 7 km/h

km/h ... avg mpg (US)

70 . . . 70.0
80 . . . 65.2
85 . . . 61.9
90 . . . 57.0

(Bi-directional averaged runs - one average per speed.)

But what do I compare it to?

1) If I look at the last time I was on the test course, it was for mirror testing. The test speed was 88 km/h. Temp was 67.6 F / 19.8 C.

88 . . . 55.2 / 56.5 - with full mirrors / pass. mirror off, driver's folded back

(Bi-directional averaged runs: 3 runs / 2 runs. These runs were also done on the same tank of gas I'm still on.)

If that's all I look at, it would appear the tray effect is good news. But. ..

2) If I look at the last time I did runs @ 70, 80, 85 & 90 km/h, it was after I changed transmissions. Temp was 73.4 F / 23 C.

70 . . . 76.8
80 . . . 65.6
85 . . . 62.2
90 . . . 58.2

(Bi-directional averaged runs - one average per speed.)

... which would lead me to conclude my MPG has dropped with the new tray!

3) Testing the partial boat tail @ 88.5 km/h / 55 mph, the temps were much warmer: Temperature ... 29°C / 84 F

88.5 . . . 61.2 / 59.9 - with boat tail on / off

(Bi-directional averaged runs - 3 runs / 2 runs.)

So!

I think all this tells me is it's impossible to compare things on different days and temperatures. A-B-A is best, but at a minimum, it needs to be an A-B done immediately before/after.

I'll try to do temperature compensation calcs later, and see what that says, but the other thing I need to consider is that (a) I possibly made things worse by adding the tray, or (b) it didn't help any!
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