Average MPG over 28,789.6 miles, with the last 1462 of those being with a ZF5 and all prior being with a c6. In teh last measurement I exlude the first tank I tracked which was 8.25 MGP caused by timing being at 6.5*, I set it to about 9* and MPG immproved from 8.25 to 10.6 on the next tank and 12.6 on the following tank.

AVG Lifetime: 11.95858517
AVG ZF5: 14.6852
AVG C6: 11.79168411
AVG C6 excl worst: 11.85611655

This is mostly around town driving. I'd guesstimate 75% stop and go and 25% highway. That's a 2.9 MPG improvement. I suspect it will be a greater improvement for purely highway miles but I haven't got a measure of strictly highway MPG yet.

For my usual driving so far at current fuel prices of $2.50/gal my fuel cost per mile went from $.21 to $.17, a $.04/mi improvement.

Cost breakdown: Note I bought the ZF5 with a seized 7.3, which I parted out and subtracted that profit from the ZF5 cost. I probably could have run it as is, but I wanted once and done and I also put a wide ratio 460 gearset in it.
Trans+engine $250.00 Rebuilt kit $173.95 Needle brg+shim $91.28 Shift lever $81.38 Shift **** $15.72 Shift bushings $14.95 Shift boot (inner) $29.95 Shift boot (outer) $64.31 Shift lever $81.38 Clutch kit $336.25 Subtotal $1,139.17

Sold parts $461.00 Net cost $678.17 Miles to ROI 16,954.25
Over the past 3 years I average 9106 miles/yr so ROI for the ZF5 swap is about 22 months from the date of the swap.

 

That's about right the over drive is about 30% and lack torque converter slip makes your numbers look about right.