Originally Posted by
MetroMPG
Not at "all speeds", but at typical road vehicle speeds.
But again, this implies that Cd is changing with speed, and that's something you never see in discussion of road vehicles at normal road speeds.
Cd is also a fudge number, which is actually averaged at various speeds in wind tunnels. Though it can be calculated, a lot of that published number takes into consideration data from actual simulations or real-world air-tunnels in addition to various factors in a calculation, and hence-forth, comes our number.
Cross-winds, which are so rarely talked about, can have a huge impact on when and where flow separation occurs. So can heading into a head-wind, where your relative velocity may be different from the airflow's. Heading into even a modest head-wind at ~10 MPH going 70 MPH on a US Interstate, means our actual airflow is nearing ~ 80 MPH. Of course, the opposite is true too with a tail wind.
I'm not going to go out on a limb and say the Cd is changing, though it could be changing at levels so minuscule it doesn't matter here, or in any published numbers.
With the way our spoilers are designed, separation is possible. Even if it isn't, there's a much better reason they reduce the angle other than "lift" as generally, if we are going to provide that spoiler with lift, it would actually help the vehicle's MPG by reducing weight on those tires, and subsequently, rolling resistance. Kind of like faking a lighter weight vehicle.
The reason spoilers are added to performance vehicles is to reduce lift, and push the vehicle's rear wheel downwards to enhance stability, handling, and traction. It costs some MPG too. We would actually benefit MPG wise from lift here.
The lift would be small anyways. So, reducing that lift is not the reason they reduced the angle.
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View my fuel log 2015 Mirage ES 1.2 automatic: 51.7 mpg (US) ... 22.0 km/L ... 4.6 L/100 km ... 62.0 mpg (Imp)