My normal mpg is 27.5. About 4 mos ago I purchased new tires P205 (one size lagers than OEM) my mpg dropped to 23. I checked my tire pressure and only had 28. Filled to 34 and my mpg returned to 26. Then the weather changed and now my mpg is 23!! I have checked and replaced my air filter and have recently had a tune-up and timing belt replaced. Could the new tires and change in weather, winter fuel reformulation really make that big of difference in my normal mpg?
All those things together could.
Changing tire sizes affects much more than you would imagine on a modern vehicle. The engine control computer uses the speed signal to calculate injector pulse width, transmission shift points, and many other engine variables. When you change the tire size without telling the computer, it throws these variables off and can reduce the performance (and mileage) of the vehicle. I would recommend that you find someone who can recalibrate the computer to the tire size (dealership can, but some tire shops might as well) and you should recover your mileage.
Grems
Did it get significantly colder? That could make the pressure in the tires go down, since air contracts when it gets colder.
The change in size is not large enough to affect computer operation. However, calculated fuel economy will be a small percentage below the actual value.
Thanks everyone for your replies. I just took a trip of 220 miles, weather was mild (48 degrees) and I obtained 29.5 mpg. So I guess it is mostly the cold weather which has been in 30’s and below. I did not know about the control computer that is calibrated to P195 OEM tire size, although it appears there is some disagreement about P205 actually affecting the calibration.
195 to 205 is unlikely to cause noticeable problems. I tossed it out there because some systems seem to be more sensitive than others.
The truth of the matter is that trying to figure these things out is like trying to nail Jello to a tree.