MPG versus exhaust backpressure

[naford]

For awhile I had an exhaust leak in the muffler and also at the spring coupler near the cat. When driving on the highway I would notice the truck would downshift when going up slight grades (slight inclines on a manual you wouldn't downshift for them). I could tell by the tach. Well I had the exhaust fixed and the truck is quiet as a mouse. But 2 things have happened one I think they disconnected the battery to weld the exhaust. The other is that the truck doesn't downshift anymore when going up slight grades. I also think gas my mpg has increased slightly. Maybe by 1/2 mpg.
Here is the question. Could a reduced or lack of exhaust backpressure reduce your mpg by causing the truck to downshift when going up slight grades? Or did the disconnecting of the battery thus reseting the computer retrain the truck to not downshift when going up slight grades? Just curoious what others think. I know that a certain amount of backpressure is needed to aid low end power. But would it be enough to help keep the truck at optimal rpm when going up slight grades. Also when the AC was running it would downshift and now it doesn't when going up slight grades.

 

[adsm08]

I would lean more towards a memory reset being responsible. 90% of your back pressure is provided by the converter and a LOSS of pressure should not cause a downshift. If anything with back pressure would cause a downshift it would more likely be excessive pressure which makes the engine work harder.

 

[RustedRanger]

I would think the exhaust leaks was probably setting off the knock sensor and reducing the timing,less timing less power,and the need to downshift having less power. And also...less timing less mpg's.