On a metro adjusting the timing is easy, Just twist the distributor housing
This will not actually do much more than move the entire spark table up or down depending on how much you move the distributor off it's factory timing. The cars are set fairly conservatively so you can usually bump the timing up 2-4 degrees with no problems. The only way to find out how much it will help is to test it out. Run a couple of tanks out at 2 degrees advanced beyond factory and if it is better than factory timing. If it is keep adding more timing and testing till your mileage goes down or you hear the car knocking.
You can pick up a few mpg and a bit more power so it is worth doing. It isn't going to make a really huge jump in your mileage unless your timing is already off from the factory settings. To do any of this you really need a timing light and follow the instructions under the hood on the emissions label to set the timing properly.
As far as the higher octane gas, I have tried it and the improved mileage was not enough to offset the cost of the gas, but it was close so it may be dependent on the car and driving style if it works out to be better.
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