I have been looking around and pricing out 35" tires. It has been my understanding that a 317/70/17 is basically a 35" x12.5 tire. So why are most tire brands that carry both a 315/70/17 and a 35x12.50/17 tire have such huge price differences between the two?

 

Usually "Load Rating" (Carcase construction). Probable rubber compound also.

 

Yep. Probably different load rating.

 

Load rating is part of it. Another part is whether the tires have a rated life and/or warranty. To get longer life, manufacturers have to use a combination of synthetic and natural rubbers. Synthetic is more expensive.

It is also more expensive for the manufacturer to make a lighter tire with the same load ratings.

Lastly, rolling resistance directly affects your gas millage. A heavier tire typically had a higher rolling resistance (lower mpg).

Another part is marketing. Tires specified as <diameter> times <width> are typically for the off-road crowd who might not be as concerned about the above issues. <metric width> / <aspect ratio> tend to be sold more into the consumer market and carry a different set of expectations.

Here's the formula for converting between the two:

- 317/70/17
Width = 317 mm

317/25.4 = 12.48 inches wide
12.48 width times .70 times two (it's a radius) plus 17 inches = 34.4720
Gives 35x12.5 r17

Going the other way:
- 33x12.5 r16
12.5 * 25.4 = 317
33 - 16 = 17
12.5/17 = .74
Gives 317/74 R16


LT317/74 R16