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Old 01-07-2021, 10:25 PM   #8 (permalink)
Master EcoModder
Join Date: Aug 2018
Location: USA
Posts: 364

Jeep - '97 Jeep Cherokee Sport
90 day: 19.36 mpg (US)

Blueberry - '07 Toyota Camry SE
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Originally Posted by COcyclist View Post
It’s complicated. I have added spats in front of my rear tires but they do not extend out beyond the body of the car. My coasting distances and tank averages are better than before I started my mods but it is very difficult to tell if the spats are helping or hurting my mpg by themselves. (I have a full under tray that may be helping the most).

It is my understanding that we want the air to stay attached down the sides of the car and detach cleanly at the back end. From the photo, it looks to me like you have added a “Gurney Flap” which is like a small spoiler that may kick the air out around the spinning wheel. I cannot say if this will help reduce aero drag without testing. It may be a good attachment point for a future fender skirt to help clear the lower edge of the tire. I hear you about the spats being difficult to remove for testing. Sometimes a compact ratcheting right angle screwdriver works. I found one cheap at a discount tool chain. You could try some tuft tests to see how turbulent the airflow is. I have seen where Julian will add a tuft taped to the very center of the wheel to see how the air flows across the wheel face.

If there is a high mpg or all electric version of your model of car it is usually safe to copy what the factory has done. FWIW some of the factory spats I have seen are about the size of a playing card, usually much smaller than the size of the front of the tire.

The “air curtains” we discuss here generally have a duct to speed the airflow across something like the front wheels so the flow is more attached down the sides after the wheel opening. Good luck. I hope his helps.
The front ones I can definitely test. I still want to do that. I think coastdown testing would be the best method, but I do have a scangauge I could use too. The rear ones would require removing the tire no matter what... there is probably 1 cm in between the tire and the screws. Not to mention I also would need to remove some body paneling to put them back on.

That is good to know that dome cars come with gurney flaps from the factory. Mine hardly stick out at all so I would think that they are beneficial. It would just be a lot of effort to test...

It is worth mentioning that I did see a slight mpg increase my last fillup, which is the first one with my mods. It has been a lot colder lately, so I think I can attribute some of the difference to my mods. I will definitely get around to testing the front ones though, and postung what I find.
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