i have a 93 F-body with M6, it has a moser 9" and 3.89 gears, which tire size would be best for me....28x12.5x15 or 26x10.5x15?

 

How much HP? Manual or automatic? Redline? Have you checked your trap speed vs. engine redline with the 3.89 gears for both of those tire diameters? Which one puts you at the top of your 1:1 gear in the traps?