What was your most succesful mod?

[mpg_numbers_guy]

Adjusting the nut behind the wheel is the #1 mod.
Getting fuel economy instrumentation is the #2 mod. It doesn't have to be a Scangauge - Torque Lite app ($0) + decent OBD2 bluetooth adapter ($22) = $22 instead of $90+.
P&G is the #3 mod. Works best with instrumentation.
Air dam is my 4th best mod, followed by tire inflation. Air dam will generally only help out a lot of the underbody of your car is unaerodynamic (most cars are).
Grille blocking and wheel covers helped some, but tested at a <1 mpg improvement for each at 40 mph. Wheel covers see greater benefit at higher speeds.

Of course there's always the boattail...but then again there are a few of us who still want to use our vehicles.

[California98Civic]

Totally agree, too, with those pointing out instrumentation, hypermiling, and tire pressure as having some of the greatest effects. Instrumentation is especially important for hypermiling because you have to monitor techniques like P&G with some precision to get them right.

[jcp123]

Yup. Driving style is what I always push first, instrumentation only if they are fairly car-nerdy. Tire inflation is a harder one to push as well.

[Ecky]

Going to need to revise my answer, a few days ago I reduced the highway cruising speed for my commute from 55mph to 50mph. Highway is only ~50% of the drive, but it the improvement appears to be approximately from 73mpg to 79mpg, which may well exceed anything else I've done, or even all of it.

I go back and forth on how much my time is worth. I expect the next time I take a road trip I'll still be driving 65, but right now I'm of the opinion that I'm not really missing that extra minute or two in the morning and afternoon. And, I'm saving about 6 cents per day extra.

[ecocruze]

Originally Posted by Ecky

Going to need to revise my answer, a few days ago I reduced the highway cruising speed for my commute from 55mph to 50mph. Highway is only ~50% of the drive, but it the improvement appears to be approximately from 73mpg to 79mpg, which may well exceed anything else I've done, or even all of it.

I go back and forth on how much my time is worth. I expect the next time I take a road trip I'll still be driving 65, but right now I'm of the opinion that I'm not really missing that extra minute or two in the morning and afternoon. And, I'm saving about 6 cents per day extra.

That's like the 5 mile drive into town I have. If I have to drive separate I always let the wife go first. Usually show up about 30-45 seconds later due to coasting into my stops

[Fat Charlie]

Nav is a very good mod, too. If you have your destination plugged in it'll give you a pretty reliable ETA (assuming no tie ups and you do the speed limit). Whatever mood I'm in and how late I feel, the nav always lets me know that I'm still on track to get there on time.