I'm not sure this is a question you can or will answer. I have an 88' suburban 350ci, TH400 trany, and 4:10 gears at back. I'm currently getting around 11-12mgp. Is it possible to get 15-16mpg at 60-65mph out of this thing. If so, what modifications should I look to make.
The best ways to improve fuel economy on any vehicle are to ensure your tires are filled to the correct tire pressure, to accelerate smoothly and slowly, and to remove any extra weight from your vehicle.
I'm also sending your question to a couple other advisors who know more about tires and transmissions, who may be able to give you better, technical, advice.
Here are some additional tips:
Hopefully these tips will bring you some improvement in your fuel economy.