I've got a 1992 Daihatsu Hijet Jumbo cab 4x4 that I've converted, starting post for the build is here
although I'll admit it's not exactly up to date. The transmission is locked in direct drive and stripped of all gears. The 4wd system is still functional but rarely used. The maximum range looks to be about 60km in good conditions which is great but more is always better. Check out my blog here for more pictures; click the electric mini truck page at the top.
In an effort to reduce drag I'm looking at using lighter fluids in the transmission, transfer case and both diffs. They currently all run standard gear oil. The transmission has no gears any more so it really just needs some light oil to keep the bearings happy. The transfer case is also direct drive when in high so there are no loaded gear meshes unless it's in 4wd. The front diff doesn't even normally spin as it has a vacuum disconnect and its rarely used but the axles turn constantly. The rear diff is of course spinning all the time and is loaded.
So I'm thinking I could use something like synthetic ATF in the front diff and transmission/t-case to keep the bearings lubed and provide some protection for when it is in 4wd, but does anyone have any experience running lighter weight oils in the rear differentials of any or rear drive vehicles? Has anyone ever had a rear diff failure? If the diff is going to last 1,000,000 km with standard gear oil I would be happy to have it wear out after 200,000km if running ATF gave me an extra 5km of range. That would be 10+ years of driving and I don't think the rest of the truck will last that long! Other ideas include lighter grease for the front CV joints.
Regardless I am going to do some tests to figure out the total drive train power losses before and after any changes. I'll just block it up, put it in 4wd and spin the wheels steady state at different speeds. In 4wd the front axles spin as well so that should be pretty close to my actual losses when rolling in 2wd. A bit more drag from the transfer case in 4wd vs 2wd but with the wheels in the air the bearing drag will be a bit lower than on the ground so I'm guessing it will equal out or be a negligible difference. Regardless it will mostly be a comparative test. My amp draw x voltage will give power losses to just turn the drive train.
My last drive where I measured consumption was 122 Wh/km from the batteries with average speeds between 60-70km/hr. That's about 136Wh/km from the wall or 217Wh/mile from the wall. Not bad already for a 4wd truck with the Aerodynamics
of a sideways brick.
What do you think? How else could I reduce parasitic drag?