bought a used truck with wrong size tires on it-sticker calls for 265 and tires were 235. i just put new tires on -correct 265's--seems like one rotation of the larger tire vs smaller tire will give me better gas mileage--is this a correct assumtion? thanks

 

I think the 235's are a smidge taller but narrower than a 265. At least that was the case with my work truck, the 265/75-16's were than the 235/85-16

 

You may or may not get better mpg with the taller tire. There are a lot more to it than simply just the diameter.