Can I Get Better Mpg From A Supra Tranny Swap

[SC4001993]

I have a 1993 SC400 and my trans is shifting hard when going from Park to Drive. The trans shifts perfect in all other gears.I was told to not change the trans fluid because it could cause the trans to start to slip, is this true, or is there something else i can do to make the trans not shift hard when going into drive. When i purchased the car the previous owner had bought a Toyota Supra trans and had not got around to installing it. So i have this trans problem my questions are can i do a trans flush and change the filter to solve my problems or i also was told to install a shift kit to make it shift in to drive the normal way.

Another Question i have is if i install the supra trans in my car will it improve my MPG since i will another gear to cruise in, cause at 80mph im almost at 3000 RPM and cruising at that RPM is killing my mpg on the highway since i travel to work about 30 miles a day. If i install the trans would i need any other parts to make it fit smoothly?

Let me no what u guy's think please right back other forums i have posted this on haven't replied back for about 3 weeks and i think this is a very good question to ask about gaining MPG from this SWAP.

thanks Jeremy