I've spent some time researching our transmission and it's fluid maintenance procedures. When it comes to changing the ATF there are 3 main options.

1.) Drain and Fill - Replaces ~ 3 Quarts
2.) Fluid Exchange via Cooling Line - Enables essentially a full flush w/o machine: ATF Exchange (using the cooling line)
3.) Flush @ Service Provider

To understand the mix condition of the ATF after drain and fill has been performed several times I made some basic assumptions and iterated.

Assumptions:
- Transmission Capacity - ~ 12 Quarts ATF
- Drain and Fill = 3 Quarts ATF Drained, 3 Quarts New ATF Added
- The drained ATF is homogeneously mixed
- ATF Expiration - 60,000 miles
- Drain and Fill Interval - 30,000 miles

My findings suggest that just using Drain and Fill every 30,000 miles as your maintenance plan for ATF will result in your fluid always containing 6.75 Qts of fluid that is expired. In other words, more than half of your fluid has 60,000 miles or more use on it.

See attachment for graph. There is a bit going on here, so allow me to try to decipher the plot.

Y-axis = Qts of ATF after Drain and Fill procedure is completed. Each column adds up to the total 12 Qt capacity. The column sections represent how many Qts have how many miles.

X-axis = Vehicle Mileage and Mileage on Fluid.

Key: You can calculate the miles on the fluid for each section color. The calculation is performed separately for each column.

So, after performing your second Drain & Fill @ 60,000 miles your fluid consist of:
6.75 Qts w/ 60,000 miles of use
2.25 Qts with 30,000 miles of use
3 Qts with 0 miles of use

After performing your fourth Drain & Fill @ 120,000 miles your fluid consist of:
3.8 Qts w/ 120,000 miles of use
1.26 Qts w/ 90,000 miles of use
1.69 Qts w/ 60,000 miles of use
2.25 Qts w/ 30,000 miles of use
3 Qts w/ 0 miles of use

That's 6.75 out of 12 Qts that will always have 60,000 miles or more use.

Let's shorten the Drain & Fill interval to every 15,000 miles. You can just half all of the mileage values on the plot

So, after performing your fourth Drain & Fill @ 60,000 miles your fluid consist of:
3.8 Qts w/ 60,000 miles of use
1.26 Qts w/ 45,000 miles of use
1.69 Qts w/ 30,000 miles of use
2.25 Qts w/ 15,000 miles of use
3 Qts w/ 0 miles of use

This trend will continue and you will always have 3.8 Qts of 12 Qts with 60,000 miles of use or more

After performing your eighth Drain & Fill @ 120,000 miles your fluid consist of:

1.2 Qts w/ 120,000 miles of use
0.41 Qts w/ 105,000 miles of use
0.53 Qts w/ 90,000 miles of use
0.71 Qts w/ 75,000 miles of use
0.95 Qts w/ 60,000 miles of use
1.26 Qts w/ 45,000 miles of use
1.69 Qts w/ 30,000 miles of use
2.25 Qts w/ 15,000 miles of use
3 Qts w/ 0 miles of use

3.8 Qts w/ 60,000 miles of use or more


So, i've decided i will be performing option #2 when i reach 60,000 miles.

2.) Drain, Fill, Pump + Repeat - Enables essentially a full flush: ATF Exchange (using the cooling line)

Thoughts?

Attached Images

 

Your assumption that the ATF expires at 60,000 miles is not true for everyone.

I had Blackstone do an analysis of my ATF at 75,000 miles and not only was it still acceptable, they say it could have gone longer.

I consider myself someone who drives "normal conditions." Good balance of highway and city. Large tires with a little bit of extra weight on my truck. Weekend trips. Offroad trips. Lots of elevation climbing daily. No towing.

Of course this interval would change for someone who drives "extreme conditions."

The nice thing about this chart is you can change your "expiration" assumption and still read it. If you assume 90,000 miles as expiration and 30,000 mile interval, then 5.06 Qts of your ATF will be expired (90,000 miles or more).

15,000 mile interval and 90,000 mile expiration: 2.14 Qts will have 90,000 miles or more.

I just did a drain and fill on my 2012 at 50k miles. My mechanic said it was a good color and saw no issues. I’m a gentle user of my rig and only add about 4k/mi per year to it. I change the oil(synthetic) every spring. I’m thinking I will start doing a drain and fill along with the oil change each year. I’m thinking the used fluid mixing annually with the new will be a better option then letting it go indefinitely as many people do to their various vehicle transmissions. Each year the new fluid will mix with the old with each D/F. While it wont ever be totally fresh it will always be good. In a few short years the fluid will be newer than it is now. Anything wrong with this logic? Seems the easiest way to solve the tranny fluid issue and give me peace of mind.

Maybe you're way over-thinking it?

I suggest computing average mixture miles in your spreadsheet. At 150k, this calculation is (2.85*150k+0.95*120k+...)/(2.85+0.95+...)=68668. on the day you change 3 quarts of fluid.

However, immediately before you change 3 quarts at 180k, the average mixture miles is 68668+30000=98668. This number is high, which is why method 2 is better at reducing overall average mixture miles since most of the fluid is changed if you buy and change 12 quarts.

When I used method 2, the last few quarts drained contained some new fluid mixed with the old. I don't know how much old fluid was changed, I'm guessing maybe 80-85%.

Yeah, could be. I've been told that a time or two.

Or one can just have the dealer exchange with all new fluid.
My Toyota dealership charges me $93.00 labor to change all the fluid with their machine when I buy the Toyota fluid from them. They have the best deal on that too.

I paid the dealer to do it, there was a guy on here that messed his transmission up doing it himself.
I got the flush/ fill for like $130 or so, was worth it to me and no ill effects from the flush.
Some say don't flush just drain and fill. Lots of opinions.
I had it done at like 144,000 miles, wish I'd done it a time or two before.

Drain and fill helps by conditioning the overall atf fluid. Also when the oil at the bottom of the pan is drained it takes with it some (not all) of the particle matter. Never the less full atf exchange is better.

Bringing it to the dealer every 60k for like $150 seems way easier.

Don’t be that guy. The DIY fluid exchange is super easy to do and hard to screw up. Just measure what you pump out, lather, rinse, repeat. Takes less than 1/2 hour to do, which is less time you’ll be spending to/from and waiting at the dealer.

At 84 years old, I am going to be that guy😁

Interesting stuff in trannies in these by Car Care Nut

How to make your Toyota Last Over 300k Miles without Major Repairs - YouTube

NEVER do THIS to your Automatic Transmission Car - YouTube

I'm 35 and I'm that guy. I dropped a pan once to do a fluid change on a previous vehicle and it cost me $2500 for a rebuild. If it happens after the dealer does it I can point the finger to them. I'm beat if I did it. No more trans services for me especially when they are a relatively inexpensive service ticket.