Wheel spacer help

[tb8125]

Hey guys I am trying to fit some wheels on my Grand National And need some help ordering wheel spacers.. My question what is the spacer compensating for offset or backspacing.. Or should I be just trying to keep the wheels off of the frame and other suspension/brakes components. I had 1" notched out the rear frame to fit 20X10 0 offset 5.5" of backspace (no rub in the back)and the front is now on 18X8 0 offset a 4.5 backspace (with no rub lock to lock) What information do I need to calculate what size spacer I need to get the new wheels to fit ...I was thinking for the rear .8 or 1" spacers and for the front 1 to 1.25" spacer
Stock
15X7-3mm offset 4" backspace front and back

NewWheels
19X8.5 20mm offset 5.5" backspace up front
20X10 20mm offset 6.3" backspace in the rear

[tb8125]

any help

[MrQuick]

Im pretty sure most are staying away from this one cause of modified parts and that you are close to the sizes you need.

Another thing to remember is that you will not have as much tire bulge with a 17"+ rim than you would with a 15" rim.

I would suggest getting your car up on stands and doing the math to be sure you are making use of all the space and not going to touch anything.
rough guidlines:
1 1/2" minimum inner clearance
1/2" minimum outer clearance
average 3/8" - 1/2" tire from rim bulge with 18"+ rim sizes

I'd see how much you can move around with the 15"s and then figure the new sizes.