i am interested in purchasing a set of bbs rk rims for my 86, but i dont know how to convert the measurements. these are the sizes i am looking at.
17x9 with 5 1/2 inch backspacing front and rear. 18-inch: 18x9 1/2 with 5 1/2-inch backspacing front, 18x10 with 6-inch backspacing rear. any help you guys could offer would be greatly appreciated. thanx

 

I'm guessing that you are looking for the offset, which is usually given in mm. There isn't quite enough information in your post to get precisely there from the width and backspacing dimensions, but if you assume that the wheel flanges are 0.5" thick you can get close enough answers for most purposes.

17 x 9 x 5.5" b/s. 10" total outside wheel width, wheel center is 5" from back side. Offset = 5.5" - 5" = 0.5" = 13mm (approx)

18 x 9.5 x 5.5" b/s. 10.5" total outside wheel width, wheel center is 5.25" from back side. Offset = 5.5" - 5.25" = 0.25" = 6mm (approx)

18 x 10 x 6.0" b/s. 11" total outside wheel width, wheel center is 5.5" from back side. Offset = 5.5" - 5.5" = 0.0" = 0mm


Norm

 

1 inch = 25.4 mm

 

Norm,
your math didn't work out on the 18x 10s
18 x 10 x 6.0" bs. 11" total outside wheel width, wheel center is 5.5" from back side. Offset = 6.0" - 5.5" = 0.5" (x25.4)= +12.7mm
The rest is good though.

May as well give my two cents on picking your rims too I guess
Go with 9.5" rims with 5.5" bs in the rear. If you do the 10's you'll have to do some fenderwell mods. They aren't that major or anything just some hammering and maybe a bit of grinder work (some people just don't like hammering their cars)
Also on the fronts you'll likely end up with some rubbing taking sharp turns with 9.5s in the front.
Just thought I'd throw that out there for you

 

thanx for the help with the conversions guys. got another question. i have been looking into this wheel issue for a while now. i have heard when you go with wider wheels you should roll the fender lips. now i have an idea what that is but if someone ciuld give me an explination, just to confirm that would be great. also with the wheels, i put a suspension techniques 1in drop kit on it, with tokico blue gas shock & struts. how will the lowering affect the clearance?