When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
If the tire width and diameter are the same it should not matter if the wheel is an 18" or 20". Is the backspacing you quoted for flared or non-flared cars? I would wait to buy wheels until you can measure off of the car.
Whether you go 15'' or 20'' your over all tire height can be the same,
of coarse you can go bigger or smaller but generally you will stay around
26''ish in the front and 27''ish in the rear. As far as back space that would
be determined on your fender flares and were you want the wheel to set .
your should do some research here with searching flared fenders and reading
about the build of each one you find, there are several nice ones I have read.
Before you're set on 20's make sure you can get the right size tire. 255/60 15s are 26.5 inch diamter if I'm correct (check on tire rack). I went with 18 rims so that I could get a trie close to the right diameter for the rear. I see too may cars with larger diameter rims but tire to small that don't fit the fender well. On the front I went with 26.25 diameter tires on 17 rims. 18 x 12 rear and 17 x 10 front.
The original tire height is 27" tall. Why not consider 19s? You can go with 19x10s in the front with a 285/35-19 and 19x12s with a 345/30-19 and be right close to that 27" overall tire height. There was a blue '69 coupe done by CARS that finished 2nd in the Optima Battery that was flared with 19s on it. The car looked real good and it certainly performed. That maybe something to consider.
Before you're set on 20's make sure you can get the right size tire. 255/60 15s are 26.5 inch diamter if I'm correct (check on tire rack). I went with 18 rims so that I could get a trie close to the right diameter for the rear. I see too may cars with larger diameter rims but tire to small that don't fit the fender well. On the front I went with 26.25 diameter tires on 17 rims. 18 x 12 rear and 17 x 10 front.
What size tires are those? What flares are those. They look good with the mud flaps.
Bee Jay