Lowwwww Geared Sub Posi Rear? (and lousy MPG too!)

[Hunter]

I bought a rusted out 71 Sub and put the posi rear in my C10 shorty truck. The Sub was a 4X2 1/2 ton 350/350 auto.
My motor is a mild 350 with a 350 auto and a little Carter square bore. I was amazed at how low this thing is geared. Its into 3rd gear by 25 mph. Plus, I get like 9-10 mpg. At 70 its screaming. Did the subs come with THAT low a gearing? I thought with the auto it would be higher.... Whussup? How do I fix it? H.