...a "ballpark" figure would do.

 

I have done both swaps ... C4 to AOD, and C4 to Tremek TKO-600.

I cannot tell you what the increase in mileage was, though, because I don't know what it was before the swap. Also, in both cases, the engine was replaced at the same time as the swap, and the rear gears, so comparing before/after mileage is apples to oranges.

I can tell you, though, that my '66 with a 289/AOD combo and 3.50 9" rear, I have achieved 22 mpg. This car runs consistent 14.3's @ 101 mph. It would probably get better mileage, had I gone with a lock-up converter.

In my '69, the 408/TKO-600, I have been able to achieve 17mpg. I figure that's pretty darned good for a carburetted, big cubic inch engine, with 354 RWHP and 416 ft. lbs. of torque (so far, best run is 13.03 @ 117mph with a TON of tire spin *LOL*).

 

Thanks for the input - 17 MPG is respecatble for that combination - the part about the tire spin is the best. My 69 has a 392 stroker in it, best MPG is 14, mild street cam (224 deg@.050 lift) pulls plenty of vacuum for the power brakes. Wide ratio TL, 3:70 gears. It's best run was a 13.7@104. No traction (BFG 235/55/16s on 16x8 vintage 40s). As a comparo, my Saleen blown '07 goes 13.7 also but @ 112mph. Also a spin fest

 

From 17 to 22 with the AODE/W, o'course the EFI might have something to do with it.