Originally Posted by Fifty150hs

Tire diameter is set. It's virtually the same no matter what diameter wheel as long as the correct tire size is used with the expected diameter wheel. 17" wheel, taller tire. 20" wheel much shorter tire. But the over all diameter remains the same. That's how the tire pressure system works and it wouldn't work if there was any difference in overall tire diameter no matter what size wheel. It's the same for the speedo. If you throw a radically different overall tire/wheel diameter at it, it will not read remotely correctly and you will likely get a tire warning light. And your speedo will be off as well.