Quote:
Originally Posted by SCNTN
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...
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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.