there is about a 2" difference between 265/75/16's and 225/75/16's. also that first number is also how wide the tire is. the wider the tire the worse your gas mileage. plus if you put larger tires on your truck you would have to correct your speedo or calculate that into your gas mileage readings some how.
225's are 29"
265's are 31.5"
285's are 33"
i thought i would also point out that putting tires on that are too small also gives the allusion that you are getting good gas mileage but you are not unless you correct the speedo for the smaller tires.

 

The smaller tires will get you better fuel economy if most of your driving is in the city or on the highway where the speed limits are pussified. When you're going stop light to stop light the smaller tires get the rig moving with less effort, and where the speed limits are low the engine gets into its most efficient range sooner and more often on the donut holes.

Out here in the west where interstate speed limits are 75 to 85, those 225 go-kart wheels will hurt more than slightly oversized tires if you spend most of your time on the interstate.

 

^^^ Wolfie was pointing out that the smaller the tire, the faster the odometer rolls up the miles, and the better the uneducated mpg-tracker feels about his driving habits/vehicle.

 

^ya. that is why i used the word allusion.

i don't think you would get better gas mileage with 285/75/16 over 225/75/16 on the highway. the 285's are wider and and would have more rolling resistance than 225's.

 

If the tires are much smaller than stock and the speedo/odo hasn't been calibrated that would explain the over inflated gas mileage calculations.

Edit: should have read to the end. I see Crazzywolfie said that above already.

 

I realized that before I ever poked the Quote button. I was just adding to the discussion that within reasonable limits and under certain driving conditions smaller tires will increase actual fuel economy. Heck, my truck had 225's on it when I bought it and they were the hot ticket for what the previous owner did with the thing. It was a work truck sporting toolboxes all around the bed and it never left the city, so those donuts saved wear and tear on the drivetrain and maximized his fuel economy, too. Inside the wheel wells they looked like donuts in punch bowls, but I'm glad he ran them to be nice to my truck before I got it.

I was getting an actual 12.5MPG around town for the month I kept those small tires on, and lost 2MPG when I bumped up to 265's. But most of my miles are on 75MPH interstates and I venture off road pretty regularly so I don't worry so much about city mileage. When your town is only a mile across there just isn't that much city driving to be done anyway!