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Downshifting an automatic

For years I have downshifted my automatic trans cars for stops, as you would a stick shift to save brakes. Generally my front pads are good for 80K miles + or -, and rear shoes up to 180K.



Thinking about fuel used (fuel injected Hondas) with closed throttle during downshifts, and considering many more RPMs, am I using more or less fuel during deceleration than I would using only service brakes for slowing and stopping?

First of all, downshifting should not be done to slow a vehicle, unless you are going down a steep hill. It should be done so you are in the right gear to accelerate out of a corner. If you’re downshifting to slow your vehicle you don’t understand the concept.

Brakes are for slowing down, transmissions are for going faster.

Downshifting, with an automatic or a manual, does not decrease fuel mileage, so you can quit worrying about that, but why do you want to use an expensive transmission to slow the car when you could use the cheap brakes instead?

I don’t get it.

Brakes are inexpensive to replace. Transmissions are not.

You’re not saving any money, and you’re potentially shortening the life of your transmission.

Why?

It’s your money.

great way to put it.you see this to often on the road

What mc said.

RE: mcparadise response My reason for asking the question had nothing to do with the merits of compression braking, rather was looking for some technical info on fuel used under those conditions.

Perhaps you have had bad experience with some Detroit type auto trans (goodness knows they’ve had their share) but having had 12 Hondas with auto trans and putting on up to 200,000 miles, have never had a spec of trans problems and have always downshifted them. Why do you think well-designed trans have “Grade Logic” technology built in that automatically will downshift on steeper grades depending % grade of decline, rpm, manifold vacuum, baro press, throttle position, etc? It is to use compression to slow the car, and save on brakes. Unless you own stock in Raybestos, or Bendix, you downshift stick shift vehicles, both trucks and cars. Automatics can do the very same thing for you.

Getting back to the origional question…with ancient carbureted engines, while decelerating with closed throttle, the higher manifold vacuum would draw more quantitiy of relatively rich mixture from the idle and transfer circuits of the carb. If you were downshifting, the higher rpms generated meant that you filled the cylinders even more times with that richer mixture. So downshifting (stick or automatics) would cause incresed fuel consumption compared to staying in high gear and totally using brakes to slow.

My question is: On newer Honda fuel injected engines, with closed throttle, and tach, MAP, Baro, speed sensors and other inputs telling the control module that you are decelerating, is the injector pulse width simply shorter or are they temporarily shut off during those conditions? If they continue injecting fuel at a given rate per intake stroke, vs reducing flow rate or even cutting it off, that would make a difference in fuel consumed under deceleration. That is my question. Would like some feedback from someone knowlegeable of newer Honda technology. Thanks

The engine will use only enough gas to maintain idle speed while you are decelerating, no matter which gear you are in or how fast the engine is turning. Some brands even shut off fuel altogether under these conditions, but I don’t know if Honda is one of them.

You will not affect the fuel mileage one way or the other by downshifting or by not downshifting. It’s strictly a personal preference. If you like doing it, continue to do so. I doubt it is causing any damage. Look at all the paddle-shift automatics out there these days. I’m sure their owners drive them like manuals.

I downshift my automatic only when descending steep grades at low speed. Other than that I leave it alone, but you can do whatever you want in your car. My brakes last a long time, so I don’t see how downshifting would save me any money. Maybe you drive faster and brake harder than I do.

I’m with McP on this one also. Brakes are cheap, easy to replace and designed to slow/stop the car. Transmissions are expensive, hard(er) to replace and designed to . . . accelerate the car to a given speed via a number of gear changes to reach desired speed with the lowest RPM. Methinks you like the downshifting stuff . . . and are looking for justification (mpg, increased brake life, whatever) to do so. I think you should do so if you like it or buy a stick, so you can shift manually. Have fun, good luck with your Honda! Rocketman

I agree with mcparadice. brakes are cheaper and a lot easier to replace than transmissions or engines.

The merit of braking with your engine aside, fuel consumption is related to the torque output of your engine. As you may know, the engine is not producing the same amount of torque (ie, the figure your car brochure quotes you) all the time. It is only producing what you need to get the car moving to the speed you’ve “selected” with your right foot on the gas pedal. When you’re downshifting to brake, the change in gear ratio means the engine rev goes up. But since torque requirement is not increased, there is no increase in fuel consumption. However, I will say that a lot of modern day cars have electronically controlled engine/fuel consumption management that dramatically reduces the amount of fuel used while on brake. For example, if you’re in “D” drive and began braking from 50 mph to a standstill, for the duration you’re braking, the engine is using less than the amount of fuel required to keep the engine idle because the momentum of the car helps to carry the car forward, or keep the pistons moving.

So all in all, there is really no difference.

But I will also add that braking with your engine is useful if you need to make an abrupt stop, such as avoiding crashing into the car in front of you.