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Old 02-03-2013, 03:37 PM   #6 (permalink)
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Originally Posted by SCNTN View Post
I haven't done a true ABA test here, but apparently something isn't right with my design. Over the last two tanks my FE has dropped about 3 MPG. I have noticed that maintaining 55 MPH sacrifices a few MPG... typically at that speed, I'll average about 33 MPG, but with the half-tonneau that has dropped to about 29-30 MPG. My last tank (with fair weather) was 402 miles @ 16.4 gallons, about 24.5 MPG. I removed the cover right after filling the tank this week, and so far I'm back up to about 33 MPG highway. Rather disappointing...
Bummer, yes, but if it were me it would eat me up not knowing why.

Two possibilities, your FE checks were wrong because of weather or other factors or the 1/2 cap really did make FE worse.
I would run a few tanks without it and then try it again and see if you get the same effect, if so it may be it was just a bit too short to break the vortex and instead just captured it like a parachute.

There is a benefit with 1/2 caps and more with full caps in general, but the improvement is often overstated, I think drag reduction is somewhere between 5-10% and this then translates to 0-5% on FE, this is low enough to be lost in daily variations and error.
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