I have a 2010 f150 wuthA 5.2 engine. Previous owner installed LT305-55R20 tires. My transmission keeps shifting up and down. Could tires be causing this problem. I get about 14 mpg and do not ahve a heavy foot in town and trips

 

Wider/Larger than stock tires decrease fuel economy and in effect gear up the drive train causing more stress on the transmission. I'd say the tires are definitely contributing to the two issues you stated.

 

My winter set of wheels/tires is just one size taller & wider than the stock set. My roads are all 55mph hill and turns here in the mountains and the transmission is always shifting between 5th & 6th. Doesn't do it nearly as much as the stock size tires/wheels.

So yes - tire size can definitely affect your truck's shifting.

 

You can try and do a KAM reset to see if that helps the tranny re-learn it's shifting strategy. Just disconnect you battery and turn the headlights on. Shut off the lights and reconnect the battery after 15 minutes.

There are also a few TSB's out for that year in regards to tranny issues. Mostly hard shifts, but may be worth investigating.

I agree with the others that larger tires can also have an effect on the shifting.