How to Change Your Mustang's Differential Fluid

How to Change Your Mustang's Differential Fluid

Last Updated August 8, 2023 | Bill Tumas

While many people remember to change the engine oil in their classic Mustangs, they often forget about changing the gear oil in their differential. Although it doesn't need to be changed as frequently, it should be swapped out every 30,000 - 40,000 miles, depending on your driving habits. Here's a quick guide on how to do just that.

What You Will Need

If you have the original eight-inch or seven-and-a-quarter-inch ring on your classic Mustang, you'll notice there's a fill plug in the rear of the car. However, there is no drain plug so the only way to completely drain the rear differential is to take it apart, pull the third member out, and take the axles all apart.

Chances are most Mustang owners don't have the time or tools to take on a massive project like that, so the best thing to do is actually to take a fill tube and use a pump to suction the old oil out and then replace it with new oil.

The plug at the back of your classic Mustang is not a drain plug, It's a fill plug. When you remove this plug, in theory, nothing should come out. If the back end somehow did get overfilled, make sure to have some sort of drain pan close by to catch any oil leakage. All you need is a half-inch ratchet that easily pops in and unscrews the plug.

The cheapest way to remove old oil from your Mustang is by using a generic fluid pump. You don't need an expensive, high-end tool for this. A basic, simple pump will do the job just fine since you won't be doing this all the time.

Drain Your Mustang Differential Oil

To start, place the tube at the bottom of the differential and pump out the old oil. If the flow feels slow, it means the oil has been in there for a while. Move the tube around to make sure you get as much oil as possible before replacing it with new oil. You won't be able to get all of it out, but you can manage to get most of the oil out this way.

For new gear oil, we recommend 80/90 to 85/140 for classic Mustangs. Depending on your differential, you may need some additives to it, but normal oil is mostly going to be fine.

Get Expert Tool Tips

Refill With New Differential Fluid

There are a couple of ways you can refill the tank. There are nozzles you can put in the tank and squirt new oil in, but since you have the pump to drain it we recommend using the same pump to fill it up. Put one tube in the tank and put a second tube in the new gear oil and then connect them both to the pump. You're going to fill it until you just see it just start to drip out. That means the tank is full, and we can reinstall the fill plug. Don't over-crank the fill plug when you reinstall it, but make sure it is snug.

Make the Most Of Your Classic Mustang

Congratulations, you're all finished changing the gear oil in the rear end of your classic Mustang. This is something a lot of people forget to do, but if you want to keep your Mustang working properly, changing fluids is a must. It's a simple job, but it'll keep your mustang running well for years to come.

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This article was researched, written, edited, and reviewed following the steps outlined in our editorial process. Learn more about CJ's editorial standards and guidelines.