Hi,
I made the switch to a T5z a couple years ago. With running 3.55 gears and 25.6" tires, I'm getting 15-17 MPG. The higher mileage occurs on longer trips. My junk is highly modified and built as a street warrior.
Hope this helps.
what is your tire size and rear end ratio? I put one in mine, with 245/60-15 and 2.79 rear end.. I get better mileage in 4th than I do overdrive.
Hooked up a vacuum gauge while driving, the engine is simply under waaay to much load(although it doesn't feel like its lagging) to cruise in overdrive.
Of course the above is assuming your carb has a good tune, which can decrease mileage just as much.