I have a big block with a wide ratio Toploader. My rear ratio is 3.70 and is adequate for where and how I drive. 60 mph is 3,000 rpm, 80 is 4,000.
I rarely drive on an interstate and took that into consideration when I installed the Toploader. However, if I were on an interstate a lot more, I'd keep the 3.70 rear and install a 5-speed.
I average 7 mpg.
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