New 3.73 gear break in miles?

I don't think miles are as important as heat cycles. Most individuals say there is no break-in period required but any aftermarket installer that I have associated with recommends to do it. I've used a few different versions through the years and all were involved with heat cycles just different methods. I guess an average of them would be do 3 or 4, 10 to 25 mile drives, without loading and keeping as much constant speed as possible. Allow complete cool downs between drives.

I think I remember with Ford Racing gears they suggested 300 mile break-in with city and highway driving. After the 300 miles, just change the fluid and you are go to go. If you are using used gears, then there is no break-in period.

I agree with the heat cycles idea.

I would say also to spend the first miles listening carefully for any noises -- on acceleration, cruise, and decelaration -- also feel for vibrations. These can be signs of a bad install or a driveshaft out of balance.

89MustangGX said:

I agree with the heat cycles idea.

I would say also to spend the first miles listening carefully for any noises -- on acceleration, cruise, and decelaration -- also feel for vibrations. These can be signs of a bad install or a driveshaft out of balance.

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good advice, i agree with the heat cycles also-the gear faces have to be "work hardened" during break in...to do that, you have to add some heat and some load.

if i remember correctly, when i installed my richmond gears (i know, i should've bought FRPP) they suggest driving 15min, 30min, then 45min with adequate cooldown time in between, at no more than 50-60mph. Then just like 89 said, LISTEN for ANY abnormal sounds during all modes of operation. if all's well, change your fluid, add your friction modifier, and have fun!

i bought used gears from a friend, put them in and drove around easy for half a day or so, didnt hear any noises, and let er rip, funnest mod yet by far

I was told by a Ford Racing tech to allow up to 500 miles just to be safe.