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What car? Without cruise control? My car doesn't downshift unless I've set it on cruise control at an intermediate speed like 55 mph and it is a long downhill slope.

I understand why: it's most efficient to drive in the highest gear even at speeds as low as 45 mph in a manual. I would imagine it's similar for automatics too.

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2006 BMW X3, yes without cruise control on. I'm not sure if it downshifts - what I meant was that the transmission adjusts itself to brake on the engine when coasting downhill, so that you don't have to brake in order to not accelerate downhill.

Interesting, I first saw this automatic trans behavior as early as about Y2K in a midrange Mercedes but only with the cruise control on, and it did actually downshift to set up the engine braking effect. I've never encountered it with cruise control off though.

Ford trucks with a Tow/Haul button enabled will do exactly what you describe though, especially if you tap the brake for an instant while going downhill.

I can vouch for this; I think this is a BMW thing. Our 2012 X3 does the same thing. If you let off the gas while going down hill, you can see the RPMs on the tach rise as the transmission downshifts. I was really pleased with the feature on a trip to Asheville last fall.

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