This might be a newbie question but, why can't you upgrade to larger tires with the OEM 4.87's?
The oem 4.87's were used to run 31's and an auto trans. Running larger tires will have the same net loss of performance on the oem 4.87's and auto trans., just as they would on any other oem tire/gear combo,
The 4.87 diff housing is different for some reason, and is why you can't run taller gears. I think all 8" ring gears are the same diameter, so it must be a width issue.
