You mentioned it going over 15 under load; is this WOT ?
Cruising down the interstate,it's not un-common to see 15 or 15.5'ish for fuel milage,
but WOT should go down in the 13's & probably 12's@high rpm's if you're stock.
when my Z was stock,,,AFR went down to 12.2 on the dyno & after bolt-on's/still was in the high 12's WOT....until it came closer to max power & started to drop.
My Tuner has mine set @13.0 across the board & peak HP@7250
These other member's helping you sound like they know their Sheet,,,so keep bumping your thread & you'll figure it out.
It's alway's helpful to put out as much imfo as possible;milage,etc...
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