What is the largest tire I can fit on a stock 20" wheel on 2008 Super Duty.

 

35" x 12" will rub at full right or left wheel turning, but not bad.

 

Depends on a few things, tire width, wheel offset, truck packages, etc.

 

wheel width and offset have a lot to do with it. As stated before it is a combination of things.

 

Do not go above 33" and 10.5 on an 09 F250 or you will be replacing hubs due to bearing failure from the increased unsprung weight of the larger tires. Ask me how I know? Ive had to replace both front hubs in last 3 years twice.