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those who believe you need to "break-in" gears say that constant speed is the biggest killer of gears over the first 500 miles. i change gear oil after 150 miles, and don't drive for more than 20 minutes at a time during the first 150 miles. i change the gear oil again at 500 miles.
i recommend not driving over 50mph for the first 150 miles.

it might also be recommended to not tow anything for the first 500 miles. then, for the first 45 towing miles, go for distances of less than 15 miles, then stop and let the diff cool before starting up again.

with all that said, i also know people that say that gear "break-in" is b.s. the thought behind "break-in" is that because you have two surfaces that wear against each other and when parts are new, those surfaces don't match up perfectly. the heat cycles during the break-in procedures are thought to allow for the surfaces to establish a perfect match and a controlled wear pattern that have the same pattern at all speeds and loads (hence varying the speed for 500 miles).

the people who think you don't have to "break-in" gears say that the metal on the gears has already been heat-treated from the factory, and that they were heated a hell of a lot hotter than you driving on the gears can make them. essentially, they say the heat from driving on the gears isn't hot enough to matter, so no break-in is not necessary.

i do it anyway, just to be on the safe side. i also know guys that have installed gears one day, then ran the hell out of them the next with no problems.
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