How to measure offset & backspace

[Transporter]

Hello,

I was looking on www.tirerack.com to buy some wheels and tyres for my 1972 Charger, I started by "shopping by vehicle" and once you select Dodge - 1972 the only option is the Challenger. So I wrote to the Tech help and they told me that by measuring bolt pattern, offset and backspace they could find me a wheel/tyre to fit.

I have little idea of what is offset and backspace, I understand they are certain space at the wheel base.
Is there a practical way to measure this so there is little room for mistake.

Also if I decide to buy a new rear-end http://www.summitracing.com/parts/MSR-D60DBBCABEAD/ would this measurement change?

[bull]

They are sort of the same thing. Offset is measured as zero, positive or negative from the center line to the back of the mounting surface. Offset on a muscle car will typically be negative while positive offset is what you normally see on modern cars. Backspace is basically the amount of space between the "back" of the mounting surface and the "back" of the wheel.

Also check out the thread at the top of this page: http://www.dodgecharger.com/forum/index.php/topic,63191.0.html

[Transporter]

Thank you Bull,

So this measure actually are on the wheel itself, I though it had to do more with the wheel base, what I will do is measure the wheels I have and have it send to Tirerack, then I guess with that information they can see if a bigger wheel would fit, I have a 15" Wheel, im looking for a 16"/17"