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Is there a noticable difference in the width of a R70 series vs a R75 series tire? I would like to replace my 6.70 x 15's with a 205 x R75 X 15. I seen on Coker's site they have only 205 xR70 x15's.
The width of an R70 and an R75 should be the same -- in this case 205 millimeters -- about 8 inches. The difference will be in the height. The sidewall of a 70-series tire will be 70% of the width, a 75 series will be 75% of the width. If you want the same HEIGHT in a 70 series, then you need to go wider in the width -- about 219. Of course, they don't make 219...
a 215 will be a little wider and a little shorter, while a 225 will be even wider, and a little taller.
Of further interest to you is the spare if you elect to carry one. If the size does not match what's on the ground if you get a flat on the rear you should rotate a front to the rear installing the spare (odd size) on the front. This is only important if you have posi.
I have 215/70's all around mounted on 15x6 knockoffs. The tire fits in my spare tire tub using the longer hanger bolts. This gives a slight exposure of the spare to the elements.
The Diamond Back 205/75-15 Firestone Supreme Si tire has about a 1-1/4" wider tread than the Coker 205/75-15, and doesn't look "skinny" like the Coker and the original 7.75-15's do; no rubbing under any conditions, no problems, they balance, and they're round.
I also have 215/70's all around with no problems. I would caution you to check the archives for a lot of material on Coker tires. In short, most folks think that they are junk compared to the Diamondback. I have had both, and I would not take another set of Cokers if they were free. The Diamondbacks that I have now are great.