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First, there is a HUGE!! range in rolling resistance between make and models of tires within a given tire size. So large that changing tire sizes is small potatoes. In this case, the change would be less than 2% (in RR)
Now the LT215/85R16 is slightly larger in diameter, and that may result in slightly better fuel economy, but again, small.
I don't think you'll be able to measure a difference - and what difference you might be able to measure might be the difference in the amount of tread rubber. Worn tires get better fuel economy than new ones.
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