So, I finally had an afternoon free and decided now is as good a time as any to quantify the numbers on smooth wheel covers on a Gen III Prius. It wasn't an ideal day, what with higher wind than I'd like, so I'll probably repeat this in the spring if I can find a calm day to do it.
Location:
1 mile of Champaign County Road 500E between Curtis Road and CR 1200N was used for testing. I chose this section of road for the low traffic and flatness; elevation is 704 feet at the north intersection, 703 feet at the midpoint, and 701 feet at the south intersection with CR1200N. Despite the location, I still had to redo one set of B runs due to tractor interference (it's harvest time).
Conditions:
Overcast, 59 degrees Fahrenheit. Winds ESE at 12mph. I would have preferred less windy conditions, but this is the only time I had to go out and do this. Additionally, as you'll see from the numbers, I believe windspeed increased a little over the course of the test.
Methodology:
5 sets of N-S runs, 3 with covers (A), 2 without (B), run A-B-A-B-A. I warmed up the car for 30 minutes and verified coolant temperature with Scangauge; I don't have an X-gauge for oil temperature so had to assume that half an hour was enough to bring it up to operating temperature. The car was left on between runs with cruise control set at 50mph; covers were removed and stored in the vehicle on B runs.
Results:
A1(N): 62.3mpg
A2(N): 66.9mpg
A3(N): 67.6mpg
Average: 65.6mpg
B1(N): 61.8mpg
B2(N): 67.2mpg
Average: 64.5mpg
Change: 1.1mpg (1.7%)
A1(S): 61.7mpg
A2(S): 62.1mpg
A3(S): 61.0mpg
Average: 61.6mpg
B1(S): 59.4mpg
B2(S): 60.2mpg
Average: 59.8mpg
Change: 1.8mpg (3.0%)
There you have it, an average 2.35% improvement on a car that represents "typical" low-drag optimization in the current auto market. When I do this again in the spring I'll try and find a set of the stock wheelcovers to test against.
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