Well the truck is spending a little more fuel to push the bigger tires, but it's hard to say exactly how much cause the values you are using to determine the MPG have changed. That is if you're hand calcing, which is the way you should be IMO.
Think about it like this. Say you make a trip that is 200 miles long. On a truck with stock tires the odo. will read 200 miles. Lets say you fill up at the end of the trip and you pump 12 gals. That works out to 200miles ÷ 12 gals = 16.7 MPG.
Now put 35's on the same truck and make the same trip. Your odo. will read 180 instead of 200. You're going to calculate your mileage based on a 180 mile trip, when the truck actually
drove 200 miles. It's hard to say how much more fuel the truck will need, but just for sake of discussion well say it took 12 gallons again. Now 180 miles ÷ 12 gals. = 15 MPG.
The truck will probably take a little more than 12 gallons this time though, because it is harder on the truck to push the bigger tires(especially if you're putting mud tires on it), and that will make the MPG reading even lower.
My point is, you can't really
tell how much more fuel the truck is using until you get the speedo and the odo corrected for the tires that are on the truck.
Until then, it's just not comparing apples to apples.
Hope that rediculous argument helped
I know it confused me