Rear spacing same as offset?

Is rear spacing the same as offset but in inches instead of mm? I'm confused on this concept. I'm looking to buy a set of rims for the track (slicks on the back and skinnies up front). Don't want to grind the calipers so i'm looking at Centerline Telstars but i'm not sure what to get as far as rear spacing goes. I'm looking at a 15x10 in the rear and 15x4 or 15x6 in the front.

Also, what is a good size for the front? Is there "too skinny"?

Thanks

Is rear spacing the same as offset but in inches instead of mm? I'm confused on this concept. I'm looking to buy a set of rims for the track (slicks on the back and skinnies up front). Don't want to grind the calipers so i'm looking at Centerline Telstars but i'm not sure what to get as far as rear spacing goes. I'm looking at a 15x10 in the rear and 15x4 or 15x6 in the front.

Also, what is a good size for the front? Is there "too skinny"?

Thanks

If you have a 15x8" wheel with a 0 offset, how would you figure the backspacing?

You have a wheel with an inner measurement of 8". The outer measurement of this wheel would be 9" as you have a ½" lip on the front and back to add to the total measurement.

The next step is to find the center of the wheel. If you have an 8" wheel, add 1" making it a 9" outer measurement and divide that by 2 equaling 4.5".

A wheel that has a 0 offset means that the mounting plate is in the center of the wheel. So a 15x8" wheel with a 0 offset has a 4.5" backspacing.

Example:
17X8.5 +20mm offset
8.5 + 1.0 = 9.5
9.5 ÷ 2 + 4.75
4.75 + 20mm = total backspacing of 5.53".
Reverse Example:
17X9.5 with a 6.25" backspacing.
9.5 + 1.0 = 10.5
10.5 ÷ 2 = 5.25
6.25 - 5.25 = 1.0" of offset.
1.0" = 25.4mm of offset.
Note:
Remember that 25.4mm equals 1.0-inch.
To convert inches to millimeters multiply inches by 25.4.
To convert millimeters to inches divide millimeters by 25.4.

Well not to be ignorant beucase i'm sure that info will help a lot when i figure this out.... but how do i know what offset (rear spacing) i need for the wheel to fit?

Do a "search".... there was a good post recently giving a range of acceptable offsets for various width wheel/tire combinations on a 4th Gen.... example, for 9.5" wide wheels, you need an offset in the range of 38mm (wheel/tire "very" flush with the front fender edge) to 56mm (wheel/tire sunk 0.70" inside the fender edge). You need to know the width of the wheel to pick the required offset.