I have a 1990 Ford F-150 XLT.. I was thinking about
lifting it. If I lifted it 4", what's the biggest tire I could
put on it. Also, if I lifted it 6", what's the biggest tire
I could put on it as well. ( asking two questions)
You may want to check into the safety aspect first. Prior to retirement part of my duties was teaching pursuit driving in enforcement vehicles with nailed down suspension. We also had an F 150 fitted with outriggers to show just how easy an F 150 could be rolled on it's side. It's possible to roll a stock 150 at or slightly over 30 mph. Lifting the truck can cause it to roll at an even slower speed or in a crash. Check out both tire size and safety concerns before making a decision.
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