So does the ECU compensate for tire size? Any theories?
It's possible that some cars may adjust for tires but mine doesn't. I did the same thing(185 70 14's versus 175 70 14's)and my mileage is now "short by about 5%. This is why some of my mileage figures are carried out to three decimal points sometimes because I have to add the difference into my miles driven. I figured out that it was off by driving from one county road to another and watching my odometer. All of our roads are 1 mile apart around here and after 10 miles I would be .5 miles off. I checked the speed by timing myself at 60 mph on the speedo and it showed that it took me significantly less than 60 seconds to cover the mile.
Horsepower is how hard you hit the wall, torque is how much of the wall you take with you.
I just realized something by this thread. Within the last few months I've noticed that my test vehicle is traveling at an actual 54 mph when it displays 60 mph, for example -- it keeps me out of trouble, BUT
I'm travelling fewer miles since the rotation of the tire is increased (higher RPMs of the wheel translating to a higher speed reading). My only guess is that the tires are worn down. The speed has been verified using 2 separate "your actual speed is..." radar trailers and by my wife following me and telling me, "why were you driving so slow?" -- when I set the cruise for 10-over the limit. Calibrated it versus the other car, which is pretty much spot-on from the radar trailer.
So, I guess my gaslog is inherently flawed and I've travelled fewer miles per gallon. The changes in mileage between fill-ups still can be accurate since the speed problem is a constant.