i have a 1993 Ford F150 4wd, its automatic, it has an straight 6 with 90,000 miles. i want a bigger tire. im thinking 32s or 33s but i want to know what i could fit on a stock rim. and i dont want to work the engine too hard. what would be your suggestion on a bigger tire size?
As for the rim, it's best to just give a quick call down to the local tire shop to see what they recommend as safe. However, you usually don't have to worry about that until you get up to 35"+ tires or so.
32s and 33s should do just fine for the engine. I know guys who have 38s behind their 300s, go rock crawling with them, etc., and have no issues.
It all depends on your gearing. Do you know what final drive you currently have? You'd most likely be just fine with either of those tires if you have 3.55s, or especially if you have 4.11s, but if you have 3.00s or less, you'll start to dog the engine and that's when you can cause issues.