Between 31" A/T's and 32" M/T's it's pretty significant, FWIW. I run both and on a steep mountain pass it's the difference between going up the hill in 4th gear or having to drop to 3rd. It also equates to about a 3 MPG fuel mileage decrease.

But, my Xterra is about 700 lbs overweight, so maybe on one that isn't so heavy the difference would be less.
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ChuckH
"Every morning when I wake up I know it's not going to get any better 'til I go back to sleep again!" Al Bundy