I get about 7, or 8 MPG. As the vehicle ages, it loses efficiency. Even if you spend a lot of money replacing sensors, filters, and other stuff, you can't get it to be like a brand new engine. In your case, you would probably need a custom driveshaft, and other custom stuff that is expensive. The question is, how much do you want to put into it? I had a similar situation with my father's old Grand Am. He wanted to junk it, but I had an idea that I would fix it up. Every month something else went that was expensive. I was dumping about $300 a month into it at one point until the transmission finally went. I didn't want to spend $1,200 on a new one.