If the ATF is bad, it can affect the MPG, not the MPH, but it can also affect acceleration. I noticed this in racing and also changing out the gear oils in the transfer case in front and the gear oil in the rear differential. However, the ATF has to be really bad along with the gear oils.

The issue more likely with the vehicle would be centered around the spark plugs (from the information you gave me). I would also check tire pressure.

Changing the thermostat would not have a great effect unless you are getting a code from the check engine light. Do you know when the last time the coolant was changed?

 

ok sorry MPG x mph mistake.
Made ​​changing the coolant and follow exactly the toyota manual. Tyre pressure maintained at 33
there is a premium gas here at 40 % more expensive. Maybe that is the reason. After changing sparkplus i will
make a experiment with this gas. That could be the equivalent to USA gas.
about thermostat someone here in forum talked about the sparkplug ceramic had simptoms of overheating.
in maybe the engine is running hot and the sensor do not work well .
when i have time i will put a termopar sensor ( i have one till 700 fº) glued in the engine and check the temperature to compare with tecnical manual