Anyone know if I'll experience any power loss (and/or significant mpg loss) in my 2019 Savana 3500 6.0L 6 speed if I jump up from the OEM tire size of 245/75/16 to 265/75/16s?

I tow an 8k lb travel trailer and don't want to self destruct the power train. It pulls great on Cooper all terrain tires now. I really want to step up just "1 size" and fill in the wheel well a bit. I just got a 3 inch lift.

Any suggestions? Should I even consider a 17" wheel? I don't really want to lose more than 2 mph's of accuracy in the speedometer.

 

There are plenty of online calculators that can help you figure it out.
Speedo error will be almost 4%. the difference in size is significant enough that you will notice more sluggish acceleration, and deceleration and braking.
17" wheels would lead to a harsher ride than 16" ones.

 

I put 265/75R15s on a van (225 or 235 were the factory size) to try to correct a slow speedometer (needed tires anyways) and it drove just fine.