Hey I was wondering if I could use a 31" tall but slightly skinnier metric size tire to help with gas mileage/acceleration (less rolling mass) without making it unsafe to drive. Obviously the tires wouldn't be that much skinnier, but it would cut a lot down on weight. So is it safe to run a less wide tire in the same height?

 

I'm not aware of any really common 31" tires that are skinnier than 10.50. The next size down from the 31's would be a 30x9.50R15, so you'd drop an inch in width but also an inch in height. 30's are gonna look tiny on that Runner though. The safety factor I think is still fine, I think the actual factory size of tire on a bottom-of-the-line Runner is 225/75R15, 3 sizes down from 31's, and about 2.5" skinnier. You won't really make up for that much power either just by going skinnier, it's more about the diameter of the tire than the width.

 

Just get 35's