I'm thinking about putting a 8 3/4 4.10 suregrip in my 81 d150 with a 4speed OD and I was wondering how much it would hurt my interstate milage? The motor is a stock 1bbl slant.

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64 dart GT /225
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That gives you 12.67 start-off multiplication and a 3 to 1 final drive, that is about right.
What tire size do you run on the truck and how fast do you like to drive on the open road?
DD

Load would be empty to 1500 pounds but mostly empty I run a 235 tire on the truck.

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64 dart GT /225
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i just installed 4.10 limitedslip 8 1/4 into my 80 d150 same engine/trans as yours except 295/50 15s, seems to be getting better mileage local back country roads...havent had it on interstate yet... sure like the way it woke truck up...cant wait to install larger valves/cam dutra duals and 4 bbl

keep on roddin'