There is instances where a larger diameter tire will give you better mileage. If you do lots of highway driving for instance.

The change from 235/75/15 to a 32/11.50 wont be the end of the world for ya. Be worth it for the increase in traction IMO.

 

It's not the nitrogen that gives better mileage, it's keeping your tires properly inflated. Nitrogen supposedly leaks out slower so if you rarely or never check your tire pressure like you're supposed to, then yes, you'd get slightly better mileage after an extended period.
For me, it's cheaper to check my tire pressure every week.


There's a sweet sport for tire diameter/gear ratio. The further away from that in either direction will give you worse mileage, even on the highway. In fact more likely on the highway because you're more affected by things like wind resistance.
Once that sweet spot is attained, a wider tire will give worse mileage than a narrow tire, all other things being equal (correct inflation, tire design, etc.)

 

I messed up here & in two other threads -

I meant to say DOUBLE (not halve), the driveshaft count.
So.......
1.75x driveshaft revolutions = 3.54 ratio,
2x revolutions = 4.11 etc

 

If your tires make noise going down the highway, that's gas-guzzling friction