Gas vs brakes

When I use my automatic transmission to slow my vehicle on steep downhills, the RPMs are much higher than when the engine idles because I use my brakes. The pistons are firing many more times per minute - so am i using more gas? Is the answer different for a standard transmission?

Much depends on the mystery vehicle that you are driving.
If it is a fuel-injected vehicle made in the past decade or so, the supply of fuel to the injectors is cut off when you are coasting downhill, thus resulting in no gas consumption at that point. On an older car, you are likely using more gas.

However, I am confused by one point in your post, namely, “the RPMs are much higher than when the engine idles because I use my brakes”. Your RPMs should not increase due to use of the brakes, and if brake usage causes an increase in idle speed, there could be a mechanical problem.

More than likely, you are shifting to a lower gear (as you should) on steep downgrades. Running in a lower gear will–of course–cause the engine to turn over at higher RPMS.

VDC, welcome to the 21st century. Many modern automatics would down shift when it senses that you are using your brakes while going downhill. It’s called grade logic.

I don’t guess my '09 is modern, it doesn’t downshift when the brakes are applied.
Even tho the “pistons are firing many more times per minute” it will not use more fuel than at idle, and as has been said, the fuel probably has been shut off.

Apparently my 2011 model is also not, “modern”, as it does not downshift on downgrades when I brake.

Thanks VDC. My “mystery vehicle” is a 2011… outback… so it’s fuel injected. It’s great to know that system cuts off the gas when when you coast! I’m not sure what you found confusing about my question - I meant it this way:<<When I use my automatic transmission to slow my vehicle on steep downhills, the RPMs are much higher>> than when <> … instead of downshifting Thanks

My 1985 Accord SEi had fuel cutoff, so it goes back more than a decade in some cases.
My cousin’s 2013 Sentra with CVT “downshifts” when braking downhill.

Outbackrider–My vehicle is also a 2011 Outback.
Is yours a 4-cylinder model (with CVT), or is it a 6-cylinder model?

More or less gas with or downshift slowdown or brake pedal ?
– I don’t care –
I like the drive better when it’s more under my one foot / one pedal control.
I downshift on downgrades. I downshift to slow gradually to an obvious red light or slow traffic situation. I use the engine braking shifter selection on my hybrid too.
Much better control, much easier to maintain speed.

are you physically downshifting, or putting it in neutral when you coast?

On 3 of my automatic cars, when free falling on a downhill ride, tapping the brakes makes the transmission downshift and stay in lower gear until you push the gas pedal. They included a '95 Galant, an '05 Camry and a Mazda CX-9. Even the rental Altima I drove with the CVT transmission had a similar feature and I would see the engine RPM’s go up.

Yes, many modern automatics hold a lower gear on uphills, even with throttle released, while downshift on downhills. Some of which I’ve driven include a 96 Saturn, which even rev matched on downshifts, a 03 echo, and a 08 fit.

Apparently, some manufacturers don’t offer the latest and greatest

On my manual xmission Corolla it is easy to verify the fuel cuts off when coasting in gear. As I approach a stoplight that is red, I’m going 35-40 mph say, so I put the xmssion in 3rd gear and take my foot completely off the gas. The car slows down in a smooth fashion until I’m going about 5-10 mph, when there is a noticeable change, the car no longer continues to slow down, as the ECM figures the engine will stall if it keeps the fuel injectors off, so it turns the fuel back on again. @outbackrider, not sure if this is as easy to do or even possible with an automatic, but you could try it anyways if curious about how it works on your own car.

Previous gen Sienna vans downshift automatically on steep hills while braking, I believe the 03 4 runners do as well.

Funny what cars have grade logic. 2007 Yaris sedan. You get used to it.

Think of gas use as being a direct result of the amount of work the engine needs to do rather than a result of how fast the engine is going (RPMs). If you’r maintaining a constant speed down a hill using your gears, the engine has to do only the amount of work necessary to keep itself running, and no more. As a matter of fact, if the geartrain is engaged gravity will even help with that, so even less gas will be ingested. Don’t be misled into thinking that the vacuum in the cylinders is increasing fuel use. It isn’t. Only an increase in work (or poor engine operation) increases fuel use.

In summary, your engine is doing almost zero work going downhill, so it’ll use almost zero gas. Except in those cars that automatically shut the fuel off…they’ll use zero fuel.

outbackrider what you say is confusing me also. you say " When I use my automatic transmission to slow my vehicle on steep downhills, " then you say " I use my brakes>> … instead of downshifting" Just HOW do you use the transmission to slow down if you don’t downshift?
Also, I am glad my '09 is not “modern” . I wouldn’t want it downshifting every time I went down a hill.