Cal Club Autocross Forums

My wife and I recently bought a 2014 Mazda 6. At her request, the car is automatic, and for me it is the first automatic car I have owned. So bear with me if the question below is too stupid.

If there is a long downhill section (say 2 minutes duration), that's neither too steep, nor too flat, I'd like to shift the Mazda 6 into neutral. In this way I don't need to press on the gas to maintain speed and the car is also not picking up speed. So my question is: can I do this on an automatic car without causing any damage? The Mazda 6 manual says that I shouldn't be shifting into neutral while driving. But they don't offer an explanation and may say it for general safety reasons or to prevent misuse (for example, getting on the gas while still in neutral and then shifting into gear, or shifting into reverse by mistake). So assuming I get off the gas and shift into neutral at the start of the long downhill section, then at the end of the downhill section, I shift back into D, wait for the gear to re-engage, and then get back on the gas, am I doing any harm?

Thanks,

Mihail

Aside than the mechanical sympathy aspect of sticking the car back in D at a relatively high speed, I was always taught you should never coast in N (manual or automatic) in case you need to accelerate in a hurry for an emergency.

Now, an emergency where you need to accelerate is hard to think of, but as racers, I'm sure there are scenarios!

Max Hayter wrote:Aside than the mechanical sympathy aspect of sticking the car back in D at a relatively high speed, I was always taught you should never coast in N (manual or automatic) in case you need to accelerate in a hurry for an emergency.

Now, an emergency where you need to accelerate is hard to think of, but as racers, I'm sure there are scenarios!

Hi Max,

I've heard the "accelerate in a hurry" argument, but I don't get it for a manual. If I was in a manual, I would be in top gear in such a scenario, meaning that I really can't accelerate particularly quickly unless I downshift. And I will downshift from neutral just as quickly as I would from say 6th gear. So I don't see a safety concern, at least for a manual. I know we are talking about autos, but I'd rather we stay on the topic of whether it causes any damage as opposed to whether it is a safety risk.

So you bring one good point about mechanical wear. Does the re-engagement of 6th gear from neutral (the Mazda 6 has six auto gears) after the incline is over cause any damage? We are talking about going from idle to about 2500 rpm, which is about 1700 rpm change in engine speed. Now, I admit I don't know how the clutches in auto transmissions with planetary gear sets work, but how different is this scenario in terms of wear as compared to the transmission actually making a downshift, in which case there may be a similar or larger change in engine speed?

It isn't "accelerate in a hurry" - it is about maintaining a level of control.

I do it all the time in my truck on long grades if I can maintain freeway speeds by coasting and have never experienced anything negative by going back and forth. The shock to the transmission is negligible. A much greater shock occurs when down shifting on steeper grades to maintain downhill speeds as there is no way to rev match.

George Schilling wrote:I do it all the time in my truck on long grades if I can maintain freeway speeds by coasting and have never experienced anything negative by going back and forth. The shock to the transmission is negligible. A much greater shock occurs when down shifting on steeper grades to maintain downhill speeds as there is no way to rev match.

It's reassuring to know I'm not the only one!

Benifit can depend on the technology of the transmission as well. Some of my cars would drop to near idle under costing 'in gear', some would keep the revs going but use nearly no fuel. I was shocked when my minivan actually downshifted and induced significant engine braking. I thought that was neat until I put a monitor on it and saw it burned more fuel that way than cruising on flat ground.

My old Geo played the nice trick of shutting off the injector (just one) after just about 2secs of costing in gear (manual though).

Jay W

It may not be good for some automatic transmissions?

I think I remember reading an owners manual about not towing above a certain speed or distance w/drive wheels on the ground?

I had to rebuild my Turbo 400 after doing that. (Not that I know that is what caused the trans to need rebuilding)

I would not risk it again considering what it cost to repair an automatic transmission and how much you think you are saving on gas.

Ron Tsumura wrote:It may not be good for some automatic transmissions?

I think I remember reading an owners manual about not towing above a certain speed or distance w/drive wheels on the ground?

I had to rebuild my Turbo 400 after doing that. (Not that I know that is what caused the trans to need rebuilding)

I would not risk it again considering what it cost to repair an automatic transmission and how much you think you are saving on gas.

One big difference is that when you are coasting the engine is still running and so is the oil pump in the auto gearbox, which is propelled by the engine. When a vehicle is being towed, the engine is not running and neither is the oil pump in the auto gearbox. Hence there is poor lubrication.

Jayson Woodruff wrote:Benifit can depend on the technology of the transmission as well. Some of my cars would drop to near idle under costing 'in gear', some would keep the revs going but use nearly no fuel. I was shocked when my minivan actually downshifted and induced significant engine braking. I thought that was neat until I put a monitor on it and saw it burned more fuel that way than cruising on flat ground.

My old Geo played the nice trick of shutting off the injector (just one) after just about 2secs of costing in gear (manual though).

Jay W

I'd think that all modern engines shut off the fuel supply if the throttle is closed and if the engine rpm is sufficiently above idle. So if the grade is steep enough that you can stay in Drive and not have to press on the accelerator to maintain speed, then, by all means, you should stay in D, because the car is consuming no fuel. You will get no benefit if you go to neutral because then the engine will burn some fuel to maintain idle speed. Plus you will be accelerating and you will need to use your brakes.
I am talking about a different scenario, in which the grade is not steep enough to maintain speed while in Drive. If you stay in D, you will have to press on the throttle and burn some fuel to maintain speed. However, If you shift to N, then car's speed is maintained by the grade itself and the fuel you burn is only to keep the engine running. An example might be the descent from the Sepulveda Pass down to West LA on 405. If I shift to N, I can get pretty much down to Wilshire Blvd without pressing on the gas. I can't do that if I stay in D.

I've done this for years... from econo boxes to big trucks. I do however give the throttle a blip to closer match the revs when it re-engages. My old Prizm ran about 4k rpms at 75mph, idle was 800 rpm, it was a big shock when it went back into gear following a grade like the one from Vegas down to Baker.

Very important though that you do not push the shift lock button... you can easily shove it into reverse... most cars will not allow this without the button pushed. But with many cars going to gated shifters... the buttons have been removed... and you can knock it into reverse much easier, say when your reaching for that coke in the cup holder.

The bigger/heavier the vehicle though, the bigger an issue the lack of engine braking can play (need for brakes in an emergency). Lighter cars reach maximum velocity for a given grade at lower speeds than heaver cars. On the other hand, lighter cars can also achieve coasting ability at lesser grades too.

Be advised Ca Veh code 21710 prohibits coasting.

Craig Naylor wrote: I do however give the throttle a blip to closer match the revs when it re-engages.

I tried it with mixed results. Maybe there is less of a shock when re-engaging, though 70 mph for me is close to 2000 rpm so there isn't that much of an rpm difference from idle anyway. Sometimes however, if I am still on the gas while giving the throttle blip, or if I have given a more abrupt throttle blip, the Mazda 6 transmission will think I want to downshift and will re-engage a lower gear.

Tom McDaniel wrote:Be advised Ca Veh code 21710 prohibits coasting.

Another completely unenforceable attempt to control the masses.

George Schilling wrote:

Tom McDaniel wrote:Be advised Ca Veh code 21710 prohibits coasting.

Another completely unenforceable attempt to control the masses.

Sometimes when I double-clutch shift, it can take some time between getting out of one gear and into the next. *cough*

Will Kalman wrote:

George Schilling wrote:

Tom McDaniel wrote:Be advised Ca Veh code 21710 prohibits coasting.

Another completely unenforceable attempt to control the masses.

Sometimes when I double-clutch shift, it can take some time between getting out of one gear and into the next. *cough*

Since this was an "auto" trans thread... My auto isn't a DSG... it has an old school torque converter.