What do you think is the "smartest" mod?
 

I want to know what you guys thing is the smartest mod for your car. The mod that cost the least, and does the most. i.e. grille dam, block heater, etc

EDIT: Smartest mod, not adjusting the nut. I guess this part of my original post got deleted.

Upper grill block, without a doubt -- it added ~10% to the FE and was about 10' feet of tape.

[Edit: of course the free things are excellent value! Driving techniques and pumping up your tires are both nearly free. For air pressure, you either have to pay some quarters every time, or you buy a good pump -- I got a very good hand pump. And stopping your engine at stop lights means that you use the starter more...

Brakes are use 'em or lose 'em: if you don't use 'em often enough, they rust away!]

Adjust the nut behind the wheel.

adjust the nut behind the wheel (2) nt
 

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Grill blocks are definitely a nice improvement for aero and faster warm up, but i also think an engine block heater makes a nice difference as well, especially in this cold weather.

Crap, some of my post got deleted. Guys that said, adjust the nut, read the new post.

"Adjusting the nut behind the wheel" and commuting route adjustments. Both were free. Between those mods and drive train break-in and tire break-in, I've gained close to a 20% increase in FE with my Mustang over 20K miles.

My Grille block was my best improvement besides the 3Tech economy cam and gear.

Smart driving gave me the largest improvement by far. As for "technical" mods, the best gains/price ratio is inflating the tires to MAX-10%.

I would have to say running one size up on tires w/ 44+ psi. The 255/45/18 is 3.15% larger than the stock 245/45/18. Price is almost identical. I spend 60% of my time on the road. So the larger diameter helped the mpg without any real sacrafice.
runner up is grille block.

Grille blocks are definately #1 on the bang for bucks scale. Mirrors deletions are close too.

Using a lower viscosity oil is free, and an engine kill switch is nearly so. Do 'em all!

It's hard to say since I did all of mine at about the same time... 1/2 grill block, belly pan (engine area), windshield wiper block, and passenger mirror fold in. Not only do you see a big improvement in FE with these type of mods, road noise also went down noticeably! Tack onto that the fact that the Coroplast used was (free) recycled campaign signs, and you have some very near free mods.

boat tails
 

Boat tails have shown me the greatest benefit of anything I've tried since 1980.
They can be constructed in many ways,so it's kinda hard to quantify cost/benefit.
I'll enjoy them 'til my days are over.

mcrews -

Yup, grille blocks are probably the cheapest, but when it's time for new tires, the one-up-from-stock has worked for me. Makes all the gears X% taller.

CarloSW2

Bang for the buck, I'd have to say airdam. Both of mine were free and gave about a 2 mpg gain.