Well - I ended up needing new front tires on my '97 Lumina.
Outter and Inner edgers were bald as queue balls.

Went to Belle Tire (as they had the best deals in town) and they put it up on the hoist, i saw them farting around with the front end suspension. then then called me out. I was like "Great - Here wo go"

they said my suspension was in tip-top shape, but due to the way the old tires were worn, I needed an alignment. I agreed.

Pre Alignment - both tires were toed in almost 3/4" of a degree each.

After alignemnt, steernig feels tighter and life is good.

I filled the gas tank on the way home. I use to get like 270 Miles on one tank of gas.

I got close to 330 miles this tank.

I know that Belle Tire uses nitrogen in the tires.

Is my mileage increase due to that? or is it a combiniation of:
1. alignment
2. warmer weather
3. nitrogen in the tires

I always got around 260 miles on a tank, for as long as I can remember. I am leaning to the fact that its the nitrogen, as thats the only real "change" i have made with this car.

Nitrogen influence is greatly overblown..in as mush as virtues that are associated to it are mostly fantasy: one virtue is real and factual: using nitrogen instead of air avoids entering moisture and oxygen in the tire hence maintains corrosion away from rims internals and rim seals, avoids internal condensation and ice in winter thus eliminates consequential unbalance. Period. ON top of it, if the nitrogen is simply fed into the tire after installation, then you will still keep the air already there as 1/3 of total volume in the tire at normal operating pressure. TO reap all the corrosion saving allowed by nitrogen, the tire must be inflated and deflated using nitrogen a few times before the final fill...which tire shops will not do in order to save their bottle of nitrogen for more tires.

Nitrogen is already 80 % of standard atmosphere; its molecular weight is 28 compared to the equivalent of near 29 for "air" (the average of 80% N at 28 and 19% Oxygen at 32 and 1% other gaz). Its heat transfer capacity is vitually same, and the law of expansion with temperature is just like all gaz: PV=nRT.
IN consequence, I think having the car properly aligned, specially toe in, has saved some scratching of rubber against alphalt and this could be an economy factor. Consider also proper inflation as possible factor. More clement temperatures compared to winter also helps. AFter 3 or 4 tankfuls of gas, you will get the complete picture.

See also this link
http://www.howstuffworks.com/framed.htm?parent=question594.htm&url=http://www.popularmechanics.com/automotive/sub_care_clinic/1272166.html?page=2&c=y