My 1988 S10 4x4 would regularly hit almost 30mpg with a cap on the back and my tools. Whya year is yours and it is auto or manual, 4 door ro 2 door. The older Blazers had the gas tank behind the rear axle and the newer 4 door it moved to infront of the rear axle like the trucks. It looks like from the frame drawings that you many be able to put an older rear tank under the rear of the front tank blazer ( a guy on s10 extremists forum did it with his truck and I had planned to do it with mine). You will need to look at getting the external rear mount if you don't have it already (can't just bolt it on if you don't there is a brace inside the body it bolts to). I woulod say don't put in a rear tank unless you do add the external tire for crash protection though I still see a lot of vehicles with the rear tank and they pass gov tests I'd much rather have an extra layer behind it.