Clearing codes= +8 mpg

Here's my story.
I have been sure that I needed a new hv battery and was on the way to a backyard mechanic who had one at a good price for my 2005 model with 255 k miles. I was looking forward to recovering my gas mileage back up to the mid 40's at least He offered to scan it with his factory software and said it looked okay, even good. He saw codes for the ABS and ECM that indicated they were not communicating with each other. He told me this meant I wasn't getting Regen power. That is why, I guess I had seen so much engine time on the MPD. And maybe why the engine was charging even though the soc was 7 bars!?
I was most surprised that I hadn't noticed the lack of regen on the MPD. No, I know I saw it at some point recently. Well maybe not because the strong green arrow on my blue themed MPD looked unusual on the way home where I got about 46 mpg averaging 70 mph. It was a much pleasanter ride as I could subtly sense the power plant was cruising easier. I had averaged in the high 30's on the way over to his shop 90 miles away.
Though I have "torque" it did not show any codes. Can someone tell me why?
My mileage problems date back to the installation 4 or 5 months ago of a set of tires at Sears. It dropped lower in the winter. I suspect they disconnected the ABS at some point and gave me a code and that the code must not have cleared since.
So this all set the stage for my search for an hv battery!
This source of battery/mileage problems has not been discussed that I have seen so maybe this will help somebody else!
JohnC

 

Hi MattK and PriusGuy, Thanks for the feedback. Please excuse me for the late response.
I was not aware of any rolling resistance that you seem to be describing Matt. Only that The engine ran more. On the highway it did seem to be freer somehow, just the better peaceful pleasant driving experience that I appreciated when I got the car last fall. It was never as if I had shifted into B for instance such as you are describing.
I have yet to recheck the 12v battery. Last fall it was on the low side, around 12.1 to start and I don't remember the rest; maybe 10.8 with the electrical devices loading up. I probably need to check it as the cold could affect that. I have no idea how old it is.
Added:
Today after driving around the test showed 12.2v acc on/ 11.8v ign on/ 11.5v with lights and 11.2 with defrosters added.
The tires are Hankook optima h727 p195/65.
Codes, yes but no lights other than "maintenance" which is probably due to 5k oil change warning.