LT265 vs LT275

Did you notice any change in MPG by running a larger tire?

My stock size is 275/70R18 and one thing I like about them is they seem to give me a lot of options because it's a popular size. I'm still running the stock junk but plan to stick with this size when they finally wear down enough. I ran oversize tires on my last rig for years and you get used to the speedo being off pretty quick. Just use a GPS or app on your phone to compare reality vs indicated speed and it'll be an automatic calculation in short order.

One thing to consider is your spare. One if you can fit a larger size replacement, or two if you don't I'd be careful about going too far with the spare on the rear if you have limited slip.