My Astro had it's transmission die in the fall. I slapped together some junkyard parts to get me through the winter. The junkyard trans had no lockup on the torque converter. So my mileage went down to 12-19mpg driving normal. I rarely used it and mostly it just sat around all winter unless I needed to haul something.
So a few weeks ago I put a well built transmission in it and a Diesel torque converter that has a 1000-1200rpm stall speed. The stocker was 1400-1600rpm. Right away it was obvious it was going to be better at anything below the 45mph lockup speed, the motor would only slip about 100-200rpm when unlocked. So I got this thing put together and all the bugs out of it. I went and filled up so I could get a good mileage number. I ran the tank out and only got 21mpg. I figured something had to be wrong but it was only getting around 21mpg consistently. I had to go fill up my mom's car for her the other day and I checked the mileage in it. It was down by about 25% from usual. I checked the receipts for her car that I usually wrote the odometer reading on. After trying to figure it out, it turns out the last gas station here that sold gas without ethanol in it just started selling E10 a month or two ago.
So that pretty much kills any chance of getting a good number out of this experiment. Unless Nerys can come up with a good solution for washing gas, I will just have to say ethanol typically drops mileage by 25%-30% the converter gave me my 25% back so I am back where I started
I really would love to find a reliable place to get some E0 for a few tanks to see what it would have been capable of with the setup it currently has. I bet it would have been within range of getting a 30mpg tank this summer with just a few more minor mods.
But I can say for sure that if you are ever taking an automatic transmission out and there is a lower stall converter available you should get it. The increase in mileage should be pretty substantial. It would be a lot of work swapping converters by itself but if the engine or trans has to come out it is a no brainer.