Effect of spoiler
 

Troy has a small spoiler on the trunk. It sticks up about 3 inches at its highest point. It doesn't protrude a whole lot, but I'm just wondering how much this is effecting my FE.

https://di1.shopping.com/images/di/75...00x100-0-0.jpg

I don't have any concrete facts to support this, but IMO the spoiler is there primarily for decoration. With it on or off, I suspect it doesn't have much impact on FE.

If there's a curved trunk lip, I suspect it might be helping, but if the cabin shape is non-optimal, it might just be sitting in turbulent wake, or if the cabin shape was intended originally to form a vortex cushion over the trunk before some marketroid restyler stuck the spoiler on, then it might be messing that up.

A relatively quick way to determine whether it was helping or not would be to time some coastdown tests from 65-50mph, if it coasts longer with it on it was helping.

I added a low spoiler 1-1/2 inches or so to my 88 camry. it makes the trunk lid heavy as hell, but FE wise, if anything, I gained 1/2 to 1 mpg?

what you might try is look at the car from the side and try to see where the air would separate at the roofline /rear window, it's around an 11? angle. (or you could use a long ruler perhaps to visualize the line) if you follow that line down and it would end up behind the trunk without the spoiler but with it hit the spoiler, that it could be doing some good, if that line falls before the spoiler if might hurt, if the line fals beside it it won't matter. i'm basing this "opinion" on observations of a lot of cars, it's not written as a rule anywhere.

but the 11? separation point is based on facts and can make a lot of car shapes make sense!

also do the same for the airflow coming of the C pillars as that wants to go to the inside along the same angle...

looks like it could help from the photo.
If not, maybe vortex generators (see stickt on aero) on top of the roof would help reattach the airflow, eliminate the rear window bubble, and allow the flow to roll on the window, the trunk, the spoiler, and cleanly out...