Old Navy,

My '80 300SD came with 180R80X14 tires. Even had a tag on the door frame stating that that was original size. Speedo and odo were right on with the 185R80 tires, Also right on with the 205R70 tires. Checked it once out west with road mile markers for 400 miles. Odometer read 400.6 miles. Can't get much closer than that.

I've even run a size different on the front side to side and couldn't tell the difference but I never run different size on rear side to side. Always put the larger size on the right side because road is usually lower on right side.

P E H