So if im
Correct say in an open ended diff that calla for 80-90 75-140 would increase heat resistance for towing, but probably
Cause a 2-4% drop in mpg?

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Also easy way to tell what diff you have is to
Look on your door tag, should specify axl then just google for a chart.

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In addition if i recall from memory it was rarer for post 2000 fords to use open ended so you probably want to look up your axle code and make certain.