I had a '95 Lumina car w/ the 3.1 (I'm assuming Isuzu is using the GM engine). hwy was 29 and city was 25. I'd guess you'd be looking at mid to low 20's???
Pretty reliable engine in my book. Gutless, but just keeps going. Mine had a little over 200k when we sold it and was still running excellent. It didn't use a drop of oil until the last year when the rear main seal started leaking, and that was the only problem.