It will actually help your transmission to have overdrive off...

The excessive heat generated in an automatic transmission comes from the torque converter working hard in torque multiply mode.. it is better to down shift so the torque converter is not trying to do all the work..

When you switch overdrive off, not only does it lock out going into forth gear, but the transmission ECU is much more aggressive in keeping the torque converter locked as much as possible to minimize heat build up... a good mode for mountain driving to keep transmission temps down..

Toyota also recommends turning overdrive off if towing a heavy load for the same reason..

The effective gear ratios of 4th gear, unlocked torque converter in heavy torque multiply mode and and 3rd gear, torque converter locked are about the same, but the transmission is producing a lot less heat in the latter mode..

You might want to read this about lockup torque converters....

<small>[ June 09, 2003, 11:04 PM: Message edited by: Dandeman ]</small>