Dear Tom and Ray:

I have a 1990 Mitsubishi Montero with an automatic transmission. It is
incredibly reliable and I hope to have it for many years. I recently got a
new job and my daily commute now takes me down a long, winding mountain
road -- and, naturally, back up at the end of the day.

With Overdrive "on," I pick up speed too quickly going downhill and need to
brake a lot. Uphill, the vehicle seems sluggish and shifts back and forth a
lot between third gear and Overdrive. The manual recommends switching
Overdrive off for long grades like this, but I worry that doing so will
place undue strain on the engine and transmission. Any thoughts? -- Curtis

TOM: The owner's manual is absolutely right, Curtis. We tell most people to
just leave Overdrive on and forget about it, but your commute just happens
to contain the two specific exceptions to that rule.

RAY: When you're driving down a long, steep grade, you MUST downshift to
keep the car's speed under control -- and turning off Overdrive IS
downshifting from fourth gear (Overdrive) to third gear.

TOM: Why not just use the brakes? Because under those conditions, the
brakes can overheat and fail. And that could result in your taking an
unexpectedly dangerous shortcut to work (straight down the side of this
mountain).

RAY: In fact, on a really steep hill, you may need to downshift to second
or even first gear to keep the car under control.

TOM: The other time you want to turn off Overdrive is when the vehicle is
"hunting," which is something you want to avoid. Now, before we get all
those nasty telegrams from the zealots at the National Rifle Association,
let me explain that in automotive terminology, "hunting" is when the
transmission can't figure out which gear to be in, and it keeps switching
back and forth between two gears.

RAY: When you're climbing a moderate hill, or traveling on rolling hills at
35-45 mph, you may find the transmission shifting a lot between third and
fourth gears. While this is not nearly as harmful as losing your brakes, it
can be pretty annoying. And in cases like these, you can solve the problem
by turning off Overdrive and forcing the transmission to stay in third
gear.

TOM: In neither one of these cases will turning off Overdrive have any
negative effect on your engine or transmission. Now, if you forget to turn
Overdrive back on, and get on the highway and drive 600 miles to East
Treetrunk, you'd put a little extra wear on the engine and get poorer gas
mileage, so if you turn it off, remember to turn it back on.

RAY: But poorer gas mileage still pales in comparison to taking the
"instant" route to work, doesn't it, Curtis?