I am looking into getting an f-150 lariat 2wd and want the absolute best chance @ decent mileage besides driving like a granny. Will either axle ratio be better for fuel economy?

 

The 3.55's will give you the best possible chance at better mileage. But if you tow alot the 3.73's might do a tad better. The difference between the two is very small, just a couple of hundred rpm between the two at highway speeds.

 

I went from 3.31 to 4.10.
Regular-driving MPG was not noticeably changed.
Towing MPG went up noticeably.
The difference from 5 or 6 mpg to 8 or 9 mpg is huge.

Performance in all conditions improved noticeably as well.

 

If all you are worried about is the gas mileage, the 3.55's will get you better mileage but only by a few tenths.

I paid the extra $50 to add the 3.73's in mine (3.55's come with the 20" wheels in the FX4) for when the larger tires go on down the road.

 

I don't think that the slight improvment in mpgs is worth the trade off in power/towing ability. I opted for the 3.73s, because it didn't cost any more and if I ever want to tow anything, or add bigger tires it will be nice knowing that I have them...

 

A friend of mine has the extended cab and I have a supercrew. He has the same motor with 3.55's and I have 3.73's. Empty on the highway is the only time he gets better gas mileage than me. But it also could be his driving but I would say he drives about the same as me.