Some wheel and tire tools (from member Short Circuit's post and 1010tires.com)
Wheel Offset Calculator: http://www.1010tires.com/WheelOffsetCalculator.asp
Tire Size Calculator: http://www.1010tires.com/TireSizeCalculator.asp
Wheel Offset Calculator: http://www.1010tires.com/WheelOffsetCalculator.asp
Tire Size Calculator: http://www.1010tires.com/TireSizeCalculator.asp