On my car, the wheel can turn slightly without much resistance, however the wheels are still turning at the same amount.

I think its because the wheels can turn ever so slightly without having to skid on the ground, like the tread is flexing a bit before it has to break loose and a little more resistance is felt.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record