Topic: Question about cruise control

[Shnak]

Alright, I've got this ~20km stretch of highway, on which I set the cruise to 120kph. I find that the transmission shifts down to the 4th speed a little too often to my liking, as soon as there's even a small incline. I find myself switching to manual mode and forcing it to remain in 5th gear. For most of the inclines on this stretch of highway, it easily maintains 120kph while in 5th gear, there's maybe 2 or 3 inclines that it slows down to about 110kph before reaching the top then it naturally regains the 120kph on the way down. My thinking is that the car does better on gas if always revving around 2200rpm in 5th gear even if it struggles for a couple of inclines as opposed to often downshifting into 4th gear and revving into the 3500rpm range for all these small inclines. Am I right? Or could I actually be damaging something (over the long run) by forcing the car into the 5th gear and making it struggle to maintain its 120kph speed?

We're very close to buying a house that will require this stretch of highway twice a day... so over the long run, I just want to make sure I'm not causing damage to my transmission or engine!

[airbalancer]

I think  u are over thinking this
I would just leave the cruise on

[tpl]

Or just leave it off and let the base logic in the transmission decide.   Does the transmission change down on this road if you do not use cruise control?  You suggest that it does not on most of the inclines but what about the steepest or longest one.

[Shnak]

Or just leave it off and let the base logic in the transmission decide.   Does the transmission change down on this road if you do not use cruise control?  You suggest that it does not on most of the inclines but what about the steepest or longest one.

To be honest, I've used cruise every time I've driven on this road... And to be clear, it doesn't change down because I'm forcing it into 5th gear and it has no choice but to stay in that gear. I'm just curious on what's harder on the powertrain: having multiple downshifts to 4th gear with rpms going into the mid 3000's, or occasionally struggling to maintain speed on some inclines when forced in 5th gear...

[vdk]

You're in 5th gear at 120? I like 3rd better... 

[EV Dan]

at 120 in forth it should be turning at around 3000 IIRC. No big deal. It does tend to downshift more on cruise.

[dr_spock]

You'll more likely to burn more gas than damage your drive train.  In my experience cruise control works best when the roads are flat. 

[Shnak]

You'll more likely to burn more gas than damage your drive train.  In my experience cruise control works best when the roads are flat. 

I'm assuming you mean burn more gas by letting it downshift at will while on cruise control? That's my thinking too. I just want to make sure I'm not going to cause damage before I start setting it to cruise control in 5th gear twice a day every day for the next many years...

[airbalancer]

You'll more likely to burn more gas than damage your drive train.  In my experience cruise control works best when the roads are flat. 

I'm assuming you mean burn more gas by letting it downshift at will while on cruise control? That's my thinking too. I just want to make sure I'm not going to cause damage before I start setting it to cruise control in 5th gear twice a day every day for the next many years...

If that facks up your tranny, then your car is crap 

[Juke1]

No idea what that feels like, CVT on cruise, haaa perfect world

[quadzilla]

It could be possible that in 5th you could be (slightly) lugging the engine. Also, it could be possible that in 5th you would use more gas than if the car downshifted into 4th. Do you have an instant fuel economy display that would verify this?

How great some people think it is to have really low revving engines, this is one thing I don't like about it. My Mazda6 revved really low but on any incline it always had to downshift. Same with the Sonata I rented in the spring. In my GTi unless its a real hill, the car doesn't need to downshift at all.

[wing]

^ this.

Just because it's downshifting it doesn't mean it's using more gas.  I've verified this on a few cars where actually too high of a gear used more fuel than downshifting.  I did this a lot with the Cruze ECO because the gearing was so tall, going 60km/h in 4th used more than doing it in 3rd gear.

Why not just let it downshift?  3000rpm is nothing, let the car go -- if you are that obsessed with low RPM you should have got a V8 -- or the V6 Sonata.  A 4cyl car will downshift that's life.

[Shnak]

It could be possible that in 5th you could be (slightly) lugging the engine. Also, it could be possible that in 5th you would use more gas than if the car downshifted into 4th. Do you have an instant fuel economy display that would verify this?

How great some people think it is to have really low revving engines, this is one thing I don't like about it. My Mazda6 revved really low but on any incline it always had to downshift. Same with the Sonata I rented in the spring. In my GTi unless its a real hill, the car doesn't need to downshift at all.

I only have an average fuel economy display, no instant. Next two times I go up there, I'll try cruise at 120kph forced in 5th gear, and then cruise at 120kph with transmission let in auto mode... I'll compare the readings. If there's barely no difference at all in fuel economy, I'll let it in auto mode, of course.

And my car is only a naturally aspirated 4 cylinder... I don't expect it to pull itself up inclines while in 5th gear at 2200rpm... your GTI is much lighter and has the turbo... it's not a surprise to hear it doesn't need to down shift as much... if I had the 3.3L V6 instead, I'm sure it'd be more than fine in 5th gear for all but the very steepest inclines.

[Shnak]

^ this.

Just because it's downshifting it doesn't mean it's using more gas.  I've verified this on a few cars where actually too high of a gear used more fuel than downshifting.  I did this a lot with the Cruze ECO because the gearing was so tall, going 60km/h in 4th used more than doing it in 3rd gear.

Why not just let it downshift?  3000rpm is nothing, let the car go -- if you are that obsessed with low RPM you should have got a V8 -- or the V6 Sonata.  A 4cyl car will downshift that's life.

Good info here... I kind of assumed my car would consume less fuel in 5th gear around 2500rpm, even if it was struggling up a hill, than it would in 4th gear at 3500rpm. If that's not the case, well it only makes sense to leave it in auto and let it downshift all it wants.

Actually, now that I think of it, when cruising at 120kph on flat roads, it revs in 5th gear at around 2200rpm... on inclines, still in 5th gear, it revs up to around 2500rpm. Now, I'm no mechanic so I'm not sure how that's possible, how can it be revving slighty more while in the same speed? Is that the torque converter, or something?

[quadzilla]

Lighter yes but not by much.

GTi = 3213
Sonata = 3327

I would say it has more to do with the tq curve and gearing than anything else. Since I can lay down all 200 lbs of tq before 2,000 rpm its always available. My Mazda had the V6 and it downshifted on many small hills.

Interested in your fuel economy experiment. Do report back.

[sirAQUAMAN64]

I never really downshift with the TDI unless I'm driving well under what the gear should be, but have noticed thru instant fuel economy going up steep hills around 100km/h in 6th uses more fuel than dropping to 5th. Similarly cruising lightly at 70km/h in 4th is sometimes more fuel efficient than 5th.

I don't think downshifting is a problem for the car or tranny, it's just irritating. I had an early version of the latest generation Saturn VUE V6 and it constantly hunted gears both on the 401 and on country roads. Extremely dissatisfying.

[Minou]

^ this.

Just because it's downshifting it doesn't mean it's using more gas.  I've verified this on a few cars where actually too high of a gear used more fuel than downshifting.  I did this a lot with the Cruze ECO because the gearing was so tall, going 60km/h in 4th used more than doing it in 3rd gear.

Why not just let it downshift?  3000rpm is nothing, let the car go -- if you are that obsessed with low RPM you should have got a V8 -- or the V6 Sonata.  A 4cyl car will downshift that's life.

Good info here... I kind of assumed my car would consume less fuel in 5th gear around 2500rpm, even if it was struggling up a hill, than it would in 4th gear at 3500rpm. If that's not the case, well it only makes sense to leave it in auto and let it downshift all it wants.

Actually, now that I think of it, when cruising at 120kph on flat roads, it revs in 5th gear at around 2200rpm... on inclines, still in 5th gear, it revs up to around 2500rpm. Now, I'm no mechanic so I'm not sure how that's possible, how can it be revving slighty more while in the same speed? Is that the torque converter, or something?

Likely your torque converter unlocking and giving those couple hundred extra RPM's before a full downshift in 4th.

I would cancel the cruise on those steeper uphill sections where it would downshift in 4th, and keep the throttle constant so that it stays in 5th while loosing some speed maybe toward 110.  At those speeds, it shouldn't struggle to maintain 5th, it wouldn't constantly downshift, would reduce wear on the transmission and fuel economy would be maximized.

Some cruise controls are too sensitive about maintaining the set speed hence the frequent downshifts on the slightest of inclines and even overpasses.  They should allow for more "play" to avoid this.  Like a good driver with a steady throttle might do.

[airbalancer]

I never really downshift with the TDI unless I'm driving well under what the gear should be, but have noticed thru instant fuel economy going up steep hills around 100km/h in 6th uses more fuel than dropping to 5th. Similarly cruising lightly at 70km/h in 4th is sometimes more fuel efficient than 5th.

I don't think downshifting is a problem for the car or tranny, it's just irritating. I had an early version of the latest generation Saturn VUE V6 and it constantly hunted gears both on the 401 and on country roads. Extremely dissatisfying.

found that to be the problem when I had a truck with a 4.8L V8, moved up to a 5.3 , no more hunting

[jamie1]

Let the automatic transmission shift AS designed. Forcing it to stay in too high a gear can cause the transmission to overheat and burn the clutches. The higher engine RPM you are concerned about with never hurt an engine. Actually it is good for it.

[Trainman]

Let the automatic transmission shift AS designed. Forcing it to stay in too high a gear can cause the transmission to overheat and burn the clutches. The higher engine RPM you are concerned about with never hurt an engine. Actually it is good for it.

^  This.  We got that advice from a mechanic way back in '96.  He said the biggest issues he sees with new cars is AT transmissions (still does 15 years later).  Too many people just don't know how/when to gear down on hills as they just try to use the overdrive too long.  I have learned with the hills around here to force early downshifts, not force holding a higher gear.  I have noticed that I have to push down less on the gas pedal too, so fuel use is probably the same or maybe even less despite running at 3,500 RPM like I do with the Forester.

Of course our hills are a bit steeper and longer; that 3,500 RPM is for like 20-25 kms at 110