I have a 1971 skylark I am planing to put some 20" rims on. The car has 1" drop springs in the rear and 1"drop springs + 2" drop spindles in the front. It has disc brakes in the front. The weels are 20x10 and 20x8,5. The tires I want to run are 245/35-20 in front and 305/30-20 in the back. Will this work?
With zero backspace it can work. I have same tire height up front with same amount of lowering, but wheel is only 8 inches. It works fine except slight outward rubbing when parking. Rearwise could be trickier. My 275/40-20 (taller tire than yours) is a tight fit with zero backspace 10x20 rims. But also, I don't know how the 68-69 wheel wells differ from 70-72.
You are confusing back space with offset. Zero offset places the mounting surface of the rim at the center of it's width. An 8" rim is actually about 9" wide outside edge to outside edge. The center of that width would be at 4 1/2". That would also be 4 1/2" of back spacing. That will work just fine on the 70-72 cars. The 69 and earlier cars need about 1/2" more back space, so that would be 5" of back space on an 8" wheel. That would correlate to 1/2" of positive offset.
I finaly got around to dig out the wheels to check the back spacing. The rears have positive ET22. I thing that gives 6.4" back spacing. The fronts have positive ET18. I think this gives 5.45" back spacing.
Best back spacing on a 10" wide rim is 5 1/2- 5 5/8". That centers the rim in the rear wheel well of a 70-72 Skylark/GS. An 8" rim likes 4 1/2", so an 8 1/2" rim should be 4 3/4" BS. That should work in the front. Being that rim width is measured between the beads, the overall width of a rim is about 1" wider. That puts offset at zero, not back spacing at zero. Lowering the car can affect clearance. Rims like this are very close to the maximum widths that you can fit. The body on a car usually does not fit exactly square on the frame, so side to side differences are common.
