Quote:
Originally Posted by 74 shortbed 406
I had an 87 k10 a couple years back that cam with 3.08. I broke those axles and swapped in a set of axles with 3.42 and my mileage went up to about 16-17 from 13-14. ......
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There's your answer right there. Since a stock 305 is low on torque, trying to get it up to speeed with what may be 3.08 gears or higher is killing your mileage. You might even have 2.xx gears which really suck gas until you are going at a steady speed.
I suggest you verify what gear ratio you have, and consider going to 3.42's. They are an awesome gear for moderate low end torque, and a decent cruising rpm.