Tagging on to what Chris said, the re-gear is more dependent on terrain. For example, if you are relative flat, you can get by with a couple tire size increases before it becomes noticeable. Or, if you tow, you will notice a difference. WRT terrain, steep, think Rocky Mountain passes, you will have a problem. Or, if you are into sand running... US cars seem to come with small tires and you can bump up at least one tire size and retain close to stock performance. Much bigger and you begin to run into clearance issues. By then, re-gearing is highly recommended.

John Stewart
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