To those who have AT, do you downshift to slow down? Is there long term negative effect to the engine if you do this frequently?

I find I do this more frequently in my IS than my other cars. When I get off the freeway or when the car in front of me is getting close, I downshift as long as I know the engine won't rev too high.

 

I don't drive in manual made too often but when I do, I also downshift to slow down. I have about 8500 miles on the car now and everything still seems fine, I can't see it damaging anthing too bad. Also, the car won't let you down shift into a lower gear if your RPM's are too high.

 

You're just going to have more engine and tranny wear. That's it. Unless you're tracking the car and trying to keep the brakes from fading, or going down a long downgrade, it's always more advisable to use the brakes only.

 

increased engine and tranny wear from increasing the revs suddenly. If you want stronger braking power look into higher grip tires

 

My understanding is that you should never downshift to slow down the car. always use the brake. you downshift to be in the right gear so you can accelerate out of the corner.

 

^^ nah its more than fine doing it. you just don't want to jump 4k revs or anything

 

As said above engine braking will increase the wear on the engine/tranny vs. using brakes and wearing them.

Replacing brake Pads > Premature engine/tranny wear any day.

 

It seems that there is disagreement on this. Nevertheless, I will not do it anymore to be on the safe side. I'll just use the brakes.

One more question though, it seems that the car does not slow down when I take my foot off the gas while on the freeway (about 65 mph). I realize I can't compare the two, but my Mercedes has 7 gears (8 if you count reverse) and I notice it slowing down once I take my foot off the gas. Does this have something to do with the gear ratio and weight if the IS?

 

gear ratios have something to do with how much braking you get from engine braking, since the lower gears (higher gear ratios) will slow down the car more

When an automatic car has zero throttle, foot off the gas, is the engine still connected to the transmission? I drive a manual and when I do drive automatics I dont experience the same engine braking as I do in an automatic when I just take my foot off the gas. I use engine braking in my current car to do a lot of speed control (no downshifting, but just staying in what gear I'm currently in and braking) I dont like tapping the brakes like an auto to control the speed and when I drive an automatic it throws off my driving style

 

When going down steep grades in the mountains, they actually encourage you to use lower gears to slow your car down and to keep people from riding the brakes all the way down, which could cause the brakes to overheat and fade or fail.

I plan to drive mine like I stole it and have as much fun as legally possible.

 

Is your merc a sedan or an suv? If it's an suv, it could be more wind resistance that you're noticing. The IS has one of the lowest CD (coffecient of drag.)

 

It's an SUV. Yes, that could very well explain it. Plus my ML350 is heavier. But when I drive my C240, it slows down more than my IS. I thought it's just a Mercedes thing.

 

I use lower gears when doing down steep inclines, but I do it BEFORE I catch any sort of momentum so the car doesn't actually downshift, but rather does not upshift to a higher gear.

 

There's no proof that indicates that downshifting cause premature wear on the transmission. Downshift all you want, you'll be fine. Downshifting might not be as efficient as braking and of course you would rather wear out your brake before you wear out your tranny but there's no proof whatsoever that two everyday ordinary drivers one who downshifts and the other who brakes more often, the downshifter incurs more and costly repairs earlier or more frequently than the other person. If there is proof, let's see it.

The faster I go, the more likely I brake. The faster I need to decelerate, the more likely I brake. Other times I downshift. BOTH are just fine, drive however you wish.

My 2 cents.

 

Higher revs = more friction. More friction = more wear and tear... You don't need "proof" that is the way things work, simple physics. When you're moving there is energy, when you're slowing down you're transforming that forward motion energy into heat. The wear you save on your brakes by downshifting doens't just disappear, part of it is now being transferred to the transmission/engine. The proof is in the laws of physics.

Weather the additonal wear is of significance or is neglagable is another matter. There are too many variables to give an accurate answer, such as how often you do it, how fast you're going when you do it and how many gears you downshift.