Changing transmission fluid for the first time on an old Prius?

I bought my 2007 Prius with 175K miles on it back in July. The car was a one owner vehicle and in very good shape and well maintained. However, there was no record of transmission fluid change. Based on overwhelming suggestions on this forum I changed the transmission fluid using Toyota fluid and so far have put over 5K miles on the car with no perceivable change in mileage or anything else. I am getting in high 40's MPG and sometimes lower 50's. Still learning how to drive it for best fuel economy. I also changed spark plugs and engine oil and will change coolant in both circuits soon. As I said, the car has been performing great, but I only have 5K miles on the transmission fluid change. I am not sure it was 100% urgent because the transmission on this car is very different from your everyday automatic and is gentler on the fluid, but I do not see any harm in changing it and potential benefit. The transmission fluid will break down with time and use, so it's good to renew it.

 

How did the old fluid look, vs new?

My sense is the fluid gets most knackered in the first year and 10K miles or so, and an initial change around that time is the best thing you can do for it. Subsequent changes are less and less necessary.

 

The old fluid looked surprisingly good, actually. Did not smell burned and was not super-black. Still had some red color to it. Not as clean as the new stuff, obviously, but I was surprised by how "not bad" it was. It may be, though unlikely, that it was changed and not recorded or the record lost, but the PO was by the book kind of guy and not super technical, so I would expect it to be not changed since Toyota doesn't call for it. Assuming it was 175K mile fluid, I was very impressed by its state. I have drained automatic transmission fluid from regular automatic cars and let me tell you, they looked a lot worse than what I drained from my Prius.

 

I changed my wife's '07 transaxle fluid at 120k miles. It was mostly red. I would not expect any improvement in fuel efficiency from the fresh fluid, but I would expect the transaxle to last longer.