hi guys no matter what rear - single or two speed the ratio you have is what is !! if it was me id pull the rear cover gasket and check the ratio or count teeth /usually the ring gear is marked with some kind of #s ALSO ikm sure the 90wt rear oil should be changed or checked ? drain it and see what its like !my old 66 c-60 w/ a 366 -real short dump box would run 65 mph /when i left the quarry with a load of rock i usually made all the peterbuilt,and kenworth big dump truck snobs use the slow passing lane on the right ! i would ride there bumpers till they pulled over ! had many that would later tell others i must of had a big cubic inch hot rod motor in there,never seen an old chevy pull that hard loaded on a hill ! it was a good running 366! let the group know what happens w/ your truck and ratios!
