Recently my wife and I purchased a 2013 Toyota highlander hybrid which I absolutely love. Having been out of the ecomodding game for several years this got me excited. Plans include a roof rail delete, full belly pan, and wheel covers as the 19" alloys aren't aero friendly, but that's for future posts. The subject of my first serious ABA test is the spare tire. Toyota carries this on the undercarriage of this vehicle with a retractable cable winch to raise and lower it, just forward of the rear bumper. To stop the steel piece that holds the rim through the center of the hub from scratching the face of the rim over time, the wheel is carried face up, leaving the open back of the rim pointing down towards the road and while it is raked rearward, it doesn't look good to me (like turbulence). Not wanting to do a full delete of the spare I grabbed one of those cheap ($18) vinyl spare tire covers from the auto parts store and put it on the underside of the spare to clean up the airflow like its own belly pan, just less effective.
A quick note:
The elastic on the 27-29" spare cover was too tight to get it on and off the wheel without tools though the cover did just barely fit. I cut the elastic and using steel tie wire, fished some paracord through in its place so I could just tie it off like sweat pants and not have to fight with it; glad I did.
Now I live a bit outside of Chicago so highway traffic conditions will never be perfect for testing but I did my best to account for the important factors.
I began my trips with the tire cover on; went out then back on my test loop, then did the loop again with the cover removed, like stock. Then a third loop with the whole spare completely removed, but inside the vehicle, and finally a fourth loop with the cover installed again as the initial rain from just the first loop had stopped. For all the runs my data was taken from the factory trip mileage reading which I reset in the same places each time after reaching a target speed and setting cruise control. my loop was aprox.14 miles of I55 (from Lagrange rd. southwest to Weber if any locals are wondering) on decent pavement, light overnight traffic, and including some small hills. I stayed in the far right lane and tried to avoid any drafting.
I assumed the gains from this test would be small so as much precision as I could manage was needed. I did have the fan, radio, and heated seat on, but at the same settings for all runs.
The run data:
A1,
cover on. 35F, wind S @3.5mph.
28.7mpg @ 61mph; light rain, light traffic
A2
cover on. 33F, wind S @ 5.8mph
31.0mpg @ 60.5mph had to coast and speed back up in once place but good traffic otherwise. light rain
Cover average: 29.85
B1,
cover off. 37F, wind S @6.9mph
29.1mpg @61mph. rain stopped, traffic clear
B2
cover off. 34F wind south @6.9mph
30.3mpg @ 60.5mph. no rain, traffic clear. perfect run
No cover average: 29.7
C1
spare tire inside vehicle. 37F, wind s @6.9mph
28.8mpg @ 60.5mph great traffic, no rain just road mist
C2
spare inside. 33F wind south @ 5.8mph
31.6mpg @ 60.5mph. misty, briefly ended up in a truck's wake
no spare average: 30.2
A3
spare reinstalled with tire cover. 36F, wind southeast @ 6.9mph
27.8mpg @61mph. misty, no real traffic
A4
spare in cover on. 33F, wind south @5.8mph.
30.7mpg @60.5mph. foggy but no rain, ideal traffic
cover on round 2 average:29.25
so as I suspected the gains, if any, might be too small to measure at this level with this equipment.
some takeaways:
-Maybe the tire cover did help a teeny bit but due to minimal unintended drafting in the no spare test as well as very few vehicles in the final cover-on tests to "go with the flow" with may have skewed the data.
-Perhaps the no spare case creates a vortex in the rear bumper like you get in a pickup truck bed?
-Perhaps the airflow is already detached and so turbulent underneath at that point that it makes no difference?
-tuft testing is needed.