04 F150

What is the biggest sidewalls I can run on stock wheels. Just added a leveling kit and want to fill in some fender gap.

 

Probably around a set of 33's +/-

I had 305/50-20 on an '03. that's a smidgen over 32", and very close to stock.

Stick with 33's and you should be okay.

 

My previous truck I had 35" with 4.56 gears. It drove great plenty of power and still reasonable rpms at freeway speed.
got 12 mpg no matter if I drove like a granny or like I stole it, but it had a bunch of other stuff that probably affected that as well though.

 

305/70/17's on stock 17x7.5 wheels on a 3" level

 

After installing the 33.9x12.01x17 or 34x12x17 tires on my stock 17x7.5 rims with 5.5" back spacing, my tires were still tucked pretty far under my fenders with the stock fenderflares. I was very surprised how far it actually sat under the fender. I decided to go with 2" wheel spacers to give it a wider stance and a better look. As you can see in the before & after pics of the install of the 2" wheel spacers the tires are now sticking out about 1". Installing it this way actually worked out to be a benefit as when it comes time to get the truck reinspected in 2 years I can simply take the spacers off and the tires will go back under the truck allowing it to pass without any problems.