Bigger tires usually cost fuel. Not because the internet said so, because of three measurable things: more rotating mass, a taller effective gear, and often a more aggressive tread. Diameter is the part this site can put a number on. Weight and tread are the parts you read off the spec sheet.
The gearing effect
A taller tire is like swapping to a taller axle ratio. At the same indicated speed the engine is turning fewer rpm, which sounds efficient until you need to accelerate or climb. The transmission stays in a lower gear longer, and that is where the fuel goes. The speedometer error percent is the same percent the gearing changed.
A 6% taller tire (33 to 35) is a 6% taller gear. Some trucks can reprogram the axle ratio or the tire diameter in the BCM so the cluster and the shift schedule agree again. That recovers the shift logic. It does not recover the extra mass.
Weight and tread
- Rotating mass. A 35x12.50 all-terrain can weigh 20 to 30 lb more than a stock highway 275. That mass has to spin up on every pull.
- Tread. An aggressive tread flexes more and makes more noise and heat. Heat is fuel you already burned.
- Width. A wider contact patch is more rubber on the road. Rolling resistance goes up.
There is no honest single mpg number for "going to 35s." A highway 275 to a highway 285 of similar weight is a small change. A highway 275 to a 35-inch mud tire is a large one. Compare stamps on the comparison, then look up the weight of the actual SKU you would buy.
An underinflated stock tire costs more fuel than a correctly inflated plus-size in a lot of real trucks. Check the door-jamb pressure before you blame the stamp.