Recently installed a LS1 aluminum driveshaft in my '88 Firebird replacing the stock steel one. For the past year, since I got the car, I've averaged about 20mpg on the highway and 15mpg in the city. Took a recent trip and averaged 25mpg on the highway. Is this an expected result from a steel to aluminum driveshaft and may I see a similar difference in my '91 Firebird where I did the same change?

 

I wouldn't get my hopes up just yet, that is an unreasonable increase for the modification you performed.
Perhaps you might want a bit more than one tank full to tell you if there was any increase.
I'm not sayin there was not, just that more data seems necessary.