I have an interesting problem. 97 200sx. 32mpg one tank, 24mpg the next. Every now and again I'll get 2 32mpg tanks in a row, but very rarely. Almost always alternates every other tank. Same driving conditions, same gas from the same place. On the highway I get 35mpg consistantly, but as soon as I hit mixed driving, it's a crapshoot. Any suggestions?

Could be a few things...

Are you consistently filling up the gas tank to the same level?

When you hit mixed driving do you idle, or go up hills, or floor it more?