Lugnutz;
My situation is similar to yours. With an overdrive trans I can now travel at 65mph with the same RPMs as about 45-7 in direct drive 3rd gear. The issue is that unless I'm cruising along on a flat road, it takes a lot more gas pedal to keep at 65mph. I found that if I didn't change my driving habits and simply drove along at 65mph in overdrive no matter what, I got the same mileage. I expected to see a significant jump in MPG after I installed the overdrive and it didn't happen. I ended up driving around with my foot in the carb a lot more thinking I was good to go because the RPMs were lower.
The main point I'm trying to make is that being in overdrive all the time isn't necessarily the most economical. There are times (when traffic slows or going up hill) that you will see better economy by getting out of overdrive and back into direct drive. Wind resistance also plays a big part. Pushing my brick of a truck down the road 10-15mph faster requires a lot more fuel as well.
Bottom line is there isn't always a direct correlation between reduced RPMs and increased mileage. How you drive, what speed you maintain and when you shift into and out of overdrive all play a big part.
