Downshift Braking for Fuel Economy?

[Skydryv]

To my surprise, it seems that Mini recommends letting the engine overrun and (also) downshifting when braking, only releasing the clutch when at engine speeds below 1,000 rpm. I know, I know, I listened to the SAME EXACT "Car Talk" episode a previous poster on another thread referenced, wherein a lady was told that "...brakes are cheaper than transmissions..etc." Worse (or better) yet, dad taught be how to drive stick in a '64, three-on-the-tree Ford. Dad was ALL ABOUT the downshifting back then. (Full disclosure: dad's also the one who taught me how to do doughnuts in the Piggly Wiggly parking lot. Don't tell mom). Anyway, my understanding (now, that is) is that overrunning / allowing the wheels to drive the engine will make the electronic injection system provide zero (that's ZERO, my friends) fuel to the engine. What's that you say? The little, instantaneous MPG read-out says 99.9 whether you're coasting in idle or over-running? It does, indeed. But I'm not so sure that it's actually accounting for the fuel the ECU is using to keep the engine running at idle. Someone here please chime in on that one?

My epiphany on this issue came from Mini themselves, of all places. I got the Mini Ipad app and, in its "MINIMALISM Analyser" tutorial on braking, the following is written:

"Lift off early from the accelerator pedal and let the engine overrun - this means your MINI does not use any fuel. When approaching a red light, for example, change down another gear if necessary and when braking, only release the clutch at engine speeds below 1,000 rpm."

Am I missing something? Admittedly, I'm a bit perplexed by the use of "...if necessary..." in the sentence about changing down to another gear. Heck, it's not "necessary" for me to do that. The only time if find it "necessary" to downshift is when I need to (ahem) "merge" "expeditiously" (wink, wink. nudge, nudge). If anyone can clarify what Mini means here, I'd be much obliged.

The little tutorial goes on to say that "Coasting is worthwhile during longer periods of deceleration. To do so, simply release the clutch." I'm assuming they mean release the clutch when OUT of gear. Again, clarity of this point would be appreciated.

Finally, here's Mini's brake down (pun intended. Forgive me) and rank for most efficient deceleration: 1. Coasting without braking. Coasting & overrunning. 3. Overrunning 4. Overrunning & braking. 5. Coasting & braking. 6. Braking.

So, when I decelerate for a full stop, I downshift and overrun. Still, I worry that I'm causing the engine undue wear when I downshift / run higher RPMs than necessary. That's not to say that I'm popping out of sixth straight into second and redlining. Just dropping through the gears in order to keep the wheels driving the engine....as the wheels slow down, I downshift. Eventually, I'll brake --while still in gear (GASP!!)-- until just about 1k rpm and then take 'er outta gear (neutral). Otherwise, if I'm going to be able to coast a looong way down a good size hill, say, for example, one that includes a warning sign to truckers, then I just pop her outta gear and coast in idle. Am I doing this right?

Thoughts? Clarifications? Snide remarks?

[Angib]

All car's ECUs, pretty much since fuel injection started being used, cut all fuel from the engine on the overrun providing the revs are above 1200 or so - below that level fuel is supplied to that the engine will still idle stably the moment the clutch is dipped.

So that's why the mpg display goes to 99.9, because zero fuel is being used.

However using the car's inertia to turn over the 'dead' engine is still absorbing power that you previously paid for in fuel, so it's another case (exactly like when using the brakes) where no fuel doesn't mean economical or free.

I would bet that putting a car in neutral at 50mph and accepting the tiny fuel use to keep the engine idling is more efficient than leaving it in gear - providing that you use the car's inertia to roll slowly up to a stop line or, say, down to 20mph. But I can't prove it and have never seen any numbers comparing the two methods.

And note that this is not talking about normal driving as few people will approach a stop sign by coasting in neutral for 30 seconds, particularly in the UK.

[Skydryv]

Thanks for the info. But what about what Mini says in its MINIMALISM braking tutorial? My quasi-final thought on the matter is -per what's in that tutorial- overrunning does save at least some fuel, PROVIDED that you'll overrunning in order to slow down for an inevitable stop. Otherwise, if there's no need for immediate or inevitable braking, coasting is the way to go --mainly because it doesn't slow the car.

I guess, then, that my main concern is about wear and tear due to overrunning. Any thoughts on that??

Thanks, again.

[bmwr606]

there are 3 options to avoid an accident ...
1 brake before it
2 steer around it/through it
3 accelerate past/through it

if you are coasting in neutral, you have lost 1/3 of your options

i like to downshift to keep the rpms between 2k-3k rpms as i decel so the engine is in the proper gear if i need to quickly accelerate

some posters have also stated it is illegal in their state to coast in neutal, i cannot comfirm that

scott

[ExclusiveWorkshops]

you'd certainly fail your driving test in the U.K if you coasted anywhere

that aside, it makes sense to use overrun downhill to maintain speed, and just coast (dip the clutch or in neutral) to save fuel where conditions allow

brakes are cheaper than gearboxes, but downshifting isn't going to shorten the gearbox life if done correctly ie meeting the gear speed with engine speed (blip), though down shifting without doing so will load the gears more and shorten clutch life significantly aswell as decrease ride quality

don't change down through all the gears for the sake of it, just use the relevant gears for road speed, 5,3,1 etc or 6,4,2

many drivers would save more fuel by addressing the way they pull away, as thats where most of it goes, trying to get the car moving as fast as poss, then keeping unnecessary thottle applied when reading the road clearly shows you they could have backed off way before. Why accelerate towards a red light?

probably off the point...... but it all saves fuel