Your best modification

[orange4boy]

My favourite mod was removing my alternator and running on one or two deep cycle batteries depending on the distance/load. went from 20 to 26mpg.

Second best was a mod group: grille block / deflector / 2 inch airdam. went from 29 - 38 mpg

Third Tire pressure.

Timing if retarded can cost mpg. You can usually advance past stock 5 degrees especially if you don't pull trailers or have lead loafers. If your engine is knocking retard a couple degrees at a a time till it stops.

If your car has high mileage check your oxygen sensor. Mine was toast and was actually working in reverse causing both an over high and low fuel mix.

[BlackDeuceCoupe]

Originally Posted by modmonster

my passenger wing mirror... was very impressively successful. way more than the 2% that i was hoping for. i measured 17% initially but i think its more consistently like 8%...

And, I might mention, on MANY cars this is a V easy mod - the factory mirrors fold in toward the body (I guess to avoid damage in car washes and so forth) allowing you to do an A/B comparison of mileage impact!

This is kinda slick, no?

USDM CiViC

YouTube - JDM Power folding mirrors

Prius

YouTube - My Prius Auto Folding Mirrors 1

Personally, I've thought about removing my outside mirrors - I know they kill mileage at freeway speeds, but mirrors come in handy in bumper-to-bumper city traffic.

Heh!

I'm gonna have to look into these 'JDM power folding mirrors'.

Dude, the japs always keep the best for themselves, don't they!?!?!

[trebuchet03]

most significant modification to any car....

change in driver

[bgd73]

most significant change can be a driver..but more not about driving, but opening the mind to flaws over marketing lies and deceipt.Has the geas mileage on a new cars window EVER been correct?

The gear change in the saturn for example. Excellent.
bigger gear ratios for boxers is a LEGEND in same comparision....all on same fuel, hence instant hypermiler..Glad to see that option for the bigger inlines.

I like what I learned here. The big change really is the drivers mind set. Step one no doubt...Doubt it all, you may get someplace if patient...

[Who]

ScanGauge

[MazdaMatt]

Originally Posted by trebuchet03

most significant modification to any car....

change in driver

I can't afford a new driver.

[Bullockracing]

A custom computer tune to increase mileage, after cutting open the factory airbox and replacing the filter...

[Brian03cav]

probably my spark plugs. that would be the biggest. after that raising my tire pressure (lots of highway driving, helped alot) and also getting a fresh air filter when i bought car.

and the all important driver mod

[Johnny Mullet]

Grill block by far!

[RH77]

Engine Block Heater