I'm sure you have seen the "racing chips" sold on ebay that claim to increase horsepower and gas mileage for around 5 bucks.
The reality of it is that it will have different effects on different vehicles. But for less than a buck, it is worth trying for a few days to see how it works.
This is just a 20 cent resistor that can be picked up from any local Radio Shack. I had to buy a 5 pack for a buck. Be careful, because resistors have different watages, and different resistance. Do some research first to see exactly which resistor you will want.
On my 02 Jeep Grand Cherokee, I tried setting it at about 40 degrees. Drove on it for a couple of days and didn't notice much of a power increase if any, and it dragged my gas mileage from about 17.5 down to 15mpg. I pulled the resistor mod out, and everything went back to normal. I was only out 20 cents.
Then I put a 10k ohm resistor in my 97 Honda Civic. I can't remember exactly, but I think that tells the computer the air is about 32 degrees. I still can't beleive the difference it makes. It went from 36mpg to 41mpg! I have been driving on it for about a month now, and still getting 40 mpg on a daily basis. Not to mention the power increase. There is no doubt it got me a significant power increase. It is most noticable at highway speeds. Going from 75 to 85 passing is much easier now. And much better exceleration on the on-ramps.
I decided to make this post for any other sceptics out there. I don't recommend you pay 5 bucks or whatever for a 20 cent resistor off e-bay, unless you are seriously ignorant under the hood and need the "instructions" included. Seriously, installing this mod only takes about 2 minutes tops. I do recommend that you do a little research and find out the temp settings and shoot for just above freezing. Try it out. If you get an extra 5 mpg like I did, it will pay for itself in about a mile!