Cruise it or don't cruise it + what's overdrive?

[tHe_sTiG]

Hi there guys,

I'd like to know how to drive my car with optimum fuel economy and with the least wear on engine and transmission. My current VZ Exec (175 kw, 4 speed auto GM4L60) is my first car and I don't quite understand everything about it... yet.

The town I live in usually has quiet roads, so I set my cruise control at the speed limit - 60 km/h. However, the transmission goes back into 3rd when I do this. When driving normally with no cruise control on at that speed, transmission stays in highest gear.

So... why does it shift down to 3rd when in cruise? I thought lower revs = less fuel. Does it improve fuel economy? Should I only use cruise when on the highway, 80+ km/h since it does select highest gear when above 70 km/h.

I'd also like to know... what's overdrive for exactly? Is it 4th gear, or something else?

[itsArthurKomar]

i know that over drive is basicaly for towing or "sports gear"

but cruise i dunno i would like to know also

Hi there guys,

I'd like to know how to drive my car with optimum fuel economy and with the least wear on engine and transmission. My current VZ Exec (175 kw, 4 speed auto GM4L60) is my first car and I don't quite understand everything about it... yet.

The town I live in usually has quiet roads, so I set my cruise control at the speed limit - 60 km/h. However, the transmission goes back into 3rd when I do this. When driving normally with no cruise control on at that speed, transmission stays in highest gear.

So... why does it shift down to 3rd when in cruise? I thought lower revs = less fuel. Does it improve fuel economy? Should I only use cruise when on the highway, 80+ km/h since it does select highest gear when above 70 km/h.

I'd also like to know... what's overdrive for exactly? Is it 4th gear, or something else?

[greenacc]

Hey Arthur, are you trying to blow up OP's gearbox ?
Overdrive is NOT for sports or towing. It has very little power due to the high ratio, and should be used only for cruising.

Hey we are about to go up a big hill .... better gear up lol NO i don't think so.

Its a good question by the Stig tho, personally i don't like using 4th at low speed 60 - 80 as its always dropping back to third when you touch the gas. When i'm going through hills i drop it back to 3rd, engine and box both seem happier that way. Over 80 its top gear all the way.

[tHe_sTiG]

Hey Arthur, are you trying to blow up OP's gearbox ?
Overdrive is NOT for sports or towing. It has very little power due to the high ratio, and should be used only for cruising.

Hey we are about to go up a big hill .... better gear up lol NO i don't think so.

Its a good question by the Stig tho, personally i don't like using 4th at low speed 60 - 80 as its always dropping back to third when you touch the gas. When i'm going through hills i drop it back to 3rd, engine and box both seem happier that way. Over 80 its top gear all the way.

I'm just assuming that the engine and auto gearbox know what they're doing. Why else would it drop back to 3rd when cruising at low speed?

It just occured to me that the case might be because the car is trying to maintain the set speed. See if I don't use cruise and maintain my throttle position, the car will still have very minor changes in speed due to the constantly changing slope of the terrain. By using cruise, the car will maintain that speed without straining the OD by going back into 3rd.

I'm also assuming, the less the fuel consumption, the happier the engine and transmission. What I believe supports my theory is: when car is idling, fuel consumption increases significantly... and we all know long idling is not good for your car.

Now with my driving style, I'm getting 10.8 - 10.9 L/100 km between city and highway driving. City: 60km/h, highway: 80km/h. I'm going to try with cruise off in the city so it stays in 4th, and see how the fuel consumption goes.

[impulsive]

Overdrive is a term used to describe a mechanism that allows an automobile to cruise at sustained speed with reduced engine speed, leading to better fuel consumption, lower noise and lower wear.

Thanks Wikipedia.

[itsArthurKomar]

ah ok fe thnx for clarifying

Hey Arthur, are you trying to blow up OP's gearbox ?
Overdrive is NOT for sports or towing. It has very little power due to the high ratio, and should be used only for cruising.

Hey we are about to go up a big hill .... better gear up lol NO i don't think so.

Its a good question by the Stig tho, personally i don't like using 4th at low speed 60 - 80 as its always dropping back to third when you touch the gas. When i'm going through hills i drop it back to 3rd, engine and box both seem happier that way. Over 80 its top gear all the way.

[somefool]

I drive my car in 3rd (or white D) all the time if its 70 or under (suburban, hills ect)
So do all the blokes at the local automatic transmission place.
Overdrive basically means the torque convertor locks up and becomes a solid coupling, not a fluid coupling, eliminating torque convertor slip. It's used for cruising only.
The reason your cruise is keeping the trans in 3rd gear is because at 60kph in third, the engine is at its optimum efficiency. Yeh, low revs is better, but engage 4th at 60 and the engine will labour( too low revs), which wastes fuel and strains the motor. So for best efficiency, drive it in 3rd until around 70-80kph then select "D". Theoretically it saves fuel and wear on both engine and trans.

Only proof i have got to support this is that 1) I relay this from a transmission re-builder
2) dad and i have always driven the car like this and the trans oil has never been burnt or discoloured

[klampy]

does third gear ever lock up if the stick is held at 3? iv just noticed that when putting round town and get into an 80km zone, accelerate to kick it back to 2nd then slip into 3rd, and then it feels like it locks down into another gear, but its still 3rd, is that the converter locking up or something else?

car in question is a VX V6 auto.

[VYMAD]

It may drive in third at low speed with cruise on, due to the fact that the auto has three modes; Normal shift, Power shift and Cruise mode, which automatically tells the auto when the cruise is activated.
Not sure exactly what it changes, but might be what makes the auto want to run in a lower gear with cruise activated.
I use cruise ALL THE TIME in any speed zone and don't really take much notice of what gear it's in other than noticing I have to be doing over 80 km/h before it will change up to fourth, which I think might be the 'overdrive.'

[somefool]

does third gear ever lock up if the stick is held at 3? iv just noticed that when putting round town and get into an 80km zone, accelerate to kick it back to 2nd then slip into 3rd, and then it feels like it locks down into another gear, but its still 3rd, is that the converter locking up or something else?

car in question is a VX V6 auto.

Funny that, mine also does it when i strike 90 in white'D' - or 3rd on your box. (same box different sticker essentially).
maybe its half-lockup? got me beat.

[Duz108]

Pretty sure the computer handles at least some of the shift point decisions. You might notice that the car has a quicker pickup if you go from having your foot heavy on the brake then stomping hard on the throttle than if you put your foot gradually on the throttle then floor it.

Also, I never use cruise, nor do I keep it in 3rd and get 10l efficiency with peak hour traffic to and from work every morning. You will easily knock off 0.5l/100 by dropping into neutral when idling and maybe more when decelerating(mostly because it doesn't tempt you to keep giving it juice)

[Voodoo_SV6]

Cars run most efficently if you are cruising along with the revs at the Max Torque output point.

So if you cars max torque is achieved at say 2500rpm, then this is the point where the engine and gearbox have to work the least to sustain that speed. The computer controlled engine and auto tranny will aim for this when possible if just cruising with constant throttle input.

This is also the most fuel efficent point (not allowing for wind resistance etc)

[Cameron P]

when car is idling, fuel consumption increases significantly... and we all know long idling is not good for your car.

the fuel consumption average is based on how many litres/100km.. if your sitting there idling your doing 0km's but using fuel so the average goes up lol

[tHe_sTiG]

Cars run most efficently if you are cruising along with the revs at the Max Torque output point.

So if you cars max torque is achieved at say 2500rpm, then this is the point where the engine and gearbox have to work the least to sustain that speed. The computer controlled engine and auto tranny will aim for this when possible if just cruising with constant throttle input.

This is also the most fuel efficent point (not allowing for wind resistance etc)

Why is it then that the car doesn't do that when cruising along the highway? At that sort of speed, any auto tranny will shift into overdrive, something like 1500 rpm. If the point of max torque is indeed the point of max fuel efficiency and least wear, why doesn't the tranny choose a gear that allows that?

[Duz108]

Cars run most efficently if you are cruising along with the revs at the Max Torque output point.

So if you cars max torque is achieved at say 2500rpm, then this is the point where the engine and gearbox have to work the least to sustain that speed. The computer controlled engine and auto tranny will aim for this when possible if just cruising with constant throttle input.

This is also the most fuel efficent point (not allowing for wind resistance etc)

Well said

10char

[tHe_sTiG]

Hey guys, been driving for a few days now without cruise in the city, highway only. Fuel consumption reached between 10.7 - 10.6 and maintained that. I went back to using cruise again today and figure dropped to 10.5. So it seems the idea of lower revs = less fuel is not completely true.

Maybe the closer the revs to max torque (2800 rpm for VZ Alloytec), the better... though it doesn't seem right having the car constantly at 2800 rpm.

[monkeys437]

I always thought an overdrive gear was a gear with a ratio less than 1. For example with the 4 speed auto, 3rd gear ratio is 1, 4th gear it about 0.75 (i think) so 4th is an overdrive gear. (basically driveshaft is spinning faster than the engine).

[Voodoo_SV6]

Why is it then that the car doesn't do that when cruising along the highway? At that sort of speed, any auto tranny will shift into overdrive, something like 1500 rpm. If the point of max torque is indeed the point of max fuel efficiency and least wear, why doesn't the tranny choose a gear that allows that?

Unless you have a locked up torque converter, the revs will drop once you get off the go fast pedal, and again creep back up as you start to press your foot down back to the true RPM for the gear/speed you are doing.

This is better noted on a manual. If you are cruising along at 110kpm, your engine speed would be greater than 1500prm in a manual, auto - not so, it will head for idle

[tHe_sTiG]

Unless you have a locked up torque converter, the revs will drop once you get off the go fast pedal, and again creep back up as you start to press your foot down back to the true RPM for the gear/speed you are doing.

This is better noted on a manual. If you are cruising along at 110kpm, your engine speed would be greater than 1500prm in a manual, auto - not so, it will head for idle

Yes, but what I'm questioning is... shouldn't the manufacturer have configured the car to make it run at around max torque for highway speeds?

[Voodoo_SV6]

yes, when you have your foot down, otherwise no

[heyitsEnricoPallazzo]

Yes, but what I'm questioning is... shouldn't the manufacturer have configured the car to make it run at around max torque for highway speeds?

Max torque and power are calculated with the trottle wide open (wot). So if you want maximum torque at highway speeds, floor it!

[Duz108]

I like that idea