driving using "4" position instead of "D"

Just found today that wife was using “4” instead of “D” for full year of driving. (sienna 2206). How can it affect transmission or engine? I do not feel anything unusual in driving using “D”.

The manual says that “4” position enables engine braking, but it does not say that speed should drop on “4”.
Any opinions?
Thank you.

Greg

About the only thing that could suffer is gas mileage if she makes highway-speed trips where that final O/D ratio would be helpful. "4" is typically used to control descents down steep grades without cooking your brakes. It does this by unlocking the torque converter (TC), allowing the engine and transmission spin separately but using the TC to limit that slippage.

The biggest effect will be on the transmission fluid itself with the energy that is being dissipated going to heating up the fluid through shearing action (Google how a torque converter works for a more complete description). So my advice would be to have the transmission fluid changed. I wouldn't opt for a flush like many shops will push for. drain & fill, run for a few hundred miles, then drain & fill again.

Hope this helps!

Thank you very much jdmccright. I will change fluid as you say.

Greg