Most Effective Hatchback Aero Improvements
 

Hello Everyone

I have a 2005 Mazda 3 hatchback and i was wondering what you think the largest area for aero improvement would be if i could only do one mod.

I was on the fence between wheel skirts, a 24in roof extension or a diffuser. I would eventually like all 3, but what has the biggest effect on it own?

I am leaving on a 700 mile trip in a week and would like to make an improvement on my 32mpg average I'm getting at the moment.

Thank you for your input :)

Have you raised your psi to 40+? that's good for a couple of mpgs on the highway.

Also a difusser in the rear is only effective if there is a bellypan that has already smoothed out the air.

biggest improvement
 

the biggest improvement would be 24" of boat tail.It couldn't just be a roof extension.It would have to extend roof,sides,and belly.
You'd be filling in the most important part of the turbulent wake,which constitutes the greatest portion of your drag.
http://i1271.photobucket.com/albums/...titled10_9.jpg
http://i1271.photobucket.com/albums/...ad2/Scans4.jpg
http://i1271.photobucket.com/albums/...ad2/Scans2.jpg
It would be kinda like what NASA did for their van
http://i1271.photobucket.com/albums/...titled46_2.jpg
It could be built onto a receiver-hitch-mounted cargo carrier and easily installed/removed.
http://i1271.photobucket.com/albums/...ntitled1-7.jpg

Anything you can do in a week would only pay off over 700 miles. Save it for when you get back, then get serious.

mcrews suggestion is a good one. Adjusting the 'nut behind the wheel' happens after you hit the road.

I'm with aerohead — if only allowed one mod — a 24" overall extension. It would approximate to an oval per Morelli's Urban Car concept and have a ring shaped plasma actuator to pinch off the wake.

Love zoom-zooms...

Looking at the profile of the 05 Mazda 3 it seems to be pretty dialed in aero wise, anything small you do in the back will not yield much, if any, reward. Tire pressure (as high as you're comfortable with, I do 38), driving habits, as well as any kind of complete upper grill block, will be quick, cheap, and easy. Alsoa partial lower grill block, and air dam. I stuffed pipe insulation into my Honda upper grill slats,

http://i63.tinypic.com/16aquzl.jpg

it doesn't need to be smooth, it just needs to stop the air from going into the upper grill area, so it can be behind the grill for a cleaner look.

http://i63.tinypic.com/mlmpfl.jpg

These will push the air over and beside your car and avoid going through the engine bay and underneath. Your cooling system is designed to cool the car climbing a mountain fully loaded in death valley in July. If your air intake is positioned right, it may force more hot engine bay air into the intake which improves MPG's as well.

Maybe. And we don't know that the OP's trip isn't to Death Valley in September :-)

Nor are all cars designed to handle quite that much, at least judging from seeing various overheated vehicles by the roadsides of the local mountains (which are somewhat cooler than Death Valley). I wouldn't recommend a grille block in summer temps/mountains unless you have a ScanGauge or similar that will tell you the exact coolant temperature. I've certainly seen my Insight's coolant temp (with the block removed for summer) go up into the 220F range when climbing stiff grades on a hot day.