What does programming the VIN into the ECM do?

[emawoman]

We have a 2006 Pontiac Vibe FWD base, now just over 100,000 miles on it. Horrible gas mileage of usually 23 MPG in town, on the highway, or mixed driving. Best we have ever seen is 26 or 27 in the past 18 months that we've had the car, and that's only about 4 times!
We've cleaned MAF, put in K&N filter, inflated the Pirelli 17" tires to their max, put in a new O2 sensor and the mileage has not budged. We checked overdrive off and on and the RPM is lower with it on (default) so we think it must be working OK. Fuel mixture does not seem overly rich or lean. Car runs fine and very little pressure is needed on the pedal to get it moving. No codes on OBD-II reader. We have been stumped trying to figure out how to increase the gas mileage.
Last thing we have tried is a new ECM we bought online. Recall did not apply to our car since the numbers on the old ECM allegedly indicated nothing was wrong with the part. New ECM went in easily and the car seems to be running at a lower RPM with it. The CEL is on and the code indicates VIN needs to be programmed into it. No other codes and the car runs great.
We've read the threads where people had to tow their vehicles in to the dealer to get VIN reprogrammed, but ours runs without a problem except that we are still getting just 23 mpg. So, I'd like to know whether programming the VIN in will do anything for the car, in particular, will it do anything to improve gas mileage?

[lannvouivre]

Programming a new VIN won't do anything with your mileage. All it does is make it easier for someone to hook up a scanner to the car and get it to "talk" back. Did you take the car in for the ECM recall before you replaced the ECM? You may have had a defective ECM and could have had it replaced for free.

As far as the mileage goes, mine's been pretty bad and I'm nearly at 100k miles, same trim and year as yours. I know mine has worn piston rings from a dry and then a wet compression test, and that's the only thing besides the spark plugs or the front O2 sensor I would suspect is causing the loss of power and increased fuel consumption (since everything else is in working order.)

Make sure you don't inflate to the maximum amount specified on the tire sidewall, that's way too high. That's the amount it can safely hold without damage. Your tires warm up on the road, and temperature increases pressure when volume and number of molecules remain constant. I realize you're not here for information about tire car, but many people don't realize that gases have a direct relationship between temperature, pressure, volume, and moles.