Old Trans-axle oil = 7.5MPG loss

The old oil could have broken down and thickened up. Or, the crap circulating around may have caused slight increased drag/wear on bearings and the transfer chain

With my FJ, I gained 2 mg just by changing all the gear oil (Front axle, rear axle, and transfer case) to Mobil Delvac Synthetic. I did this when my FJ only had 1,200 km on it

 

Driving to the gas station, filling up, resetting the MFD, and making a trip starting with a warm engine is good for a substantial (but artificial) bump in trip fuel economy every time. Changing the transaxle oil was a good move but that isn't why you saw an increase in mileage.

 

Used transaxle fluid usually will test at a lower viscosity compared to new, so I would not expect improved mpg from a fluid change. However its good to periodically drain the old oil to get the Si and Fe out of the system. I also observe ~30K mile change intervals.

Let's see what happens to your mpg as you log more miles over the next few months.

 

Well I suppose in any experiment with many data points there will always be some "outliers" that don't make sense and get ignored.

 

13% mileage increase from tranny oil change seems a little out of the realm of reality. If the oil in there was bad enough to cause that much friction, the tranny would be trash by now.

 

LOL. Thanks! :rofl:

 

The NHW11 transaxle has about a 15% carry forward of the old oil from nooks and crannies that don't drain. No one has done a study of the NHW20 but I would be surprised if the carry forward wasn't in that range. You might want to treat the first change as a 'flush' and change again in 2-5,000 miles. Pity about not keeping a sample for analysis.

The particulate load is what causes the oil to become 'dark' and these fine particles break the film layer. Changing the oil has removed most of these fine particles so the oil goes back to being a lubricant instead of a 'lapping' compound.

Bob Wilson

 

My first oil change at 53k, an NHW11, was pretty grim and smelled of paraffin. Until we tested a 325k mile sample, it had been the record. Overall, the trend has been worse for samples from dusty areas.

Bob Wilson

 

The numbers only look bad because the MPG is so high to start with. The same % in MPG drop on a vehicle that gets 15mpg would barely be noticeable except to your wallet.

 

You may want to go on a more frequent oil change program ... say every 30k or 2-3 years. It is not that big of a deal and the potential mileage improvement ... well you've seen the evidence.

If you have a gentle hill near the house so you can do a 'cold' roll down test, you might baseline the performance at freezing. Then every winter, repeat the test and when the roll-down test shows evidence of higher drag, change the oil. Just a thought.

Bob Wilson

 

So what is the best tranny oil as I'm at 35,000 and its time ...is the factory Toyota best?

 

Well, probably the safest to use, as Toyota factory fills the car with it. Don't be surprised when you drain the fluid, that it is black

Note: always open the fill plug first. If for any reason the fill plug is stuck, seized, or otherwise buggered up, you sure don't want to find out after all the fluid is drained

Around 26 years ago, I learned that lesson the hard way

 

If you don't use Toyota fluid you risk damaging the insulation on the motor windings. Why take the chance with a multi-thousand dollar repair item.

 

Good tip!