I have heard various answers to this question. I have also heard it depends on the engine. What do you guys think? I am gonna guess 70 mph.

I always thought it was around 55Mph, when the RPMS are very low during the TCC lockup.

Sol'* correct for 2nd gear with TCC lockup, but if you were in say 3rd gear with it.... I've churned 70 MPH @ 1500-1700 RPMS and boy does that give good mileage.

With that, you could get close to 30mpg on the freeway.

I did for 1/4 tank coming back to school, but then the next day hanging out with smellbird, and all the idling, and around town jaunts we did, i got NOTHING.

Yea, around the city it'* not very efficient, especially with stop - go - stop - go - WOT - stop

Our engines are strange like that.

I know I am getting like 14 mpg, but that is in the winter, with a lot of WOT driving in the city. Should probably improve in the summer. Was thinking about changing my O2 sensor, has not been changed so that might be an issue.

For anyone with a DIC, you can pretty clearly see that all other things being equal, your best mileage is between 45-50 when the torque converter has locked up. Even with a SSEi, at that speed, the DIC on my car will read somewhere around 36mpg. Take it up to 70 and it drops to about the EPA rating of 27.
I did an experiment not long ago where I compared mileage at 70mph. I checked the mileage at 70 with no one around me and as stated above, I was getting about 27. Then I got behind a semi and drafted at 70 and was getting 33mpg.

I do not have a DIC, I am basing this MPG on how many gallons I get at the pump, compared to how many miles I travel.

well its best not to keep accelerating and deaccelerating at hiway speeds. i normally hit the cruise between 55 and 75, depending on the limit and weather. never bothered calcutating my mpg on a trip but ive seen 340 miles on a tank...