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Discussion Starter #1
The other night I was driving and I noticed that in 5th gear my car car's tachometer was at exactly 3,200 rpm. The speedometer was at 121 km/h. It got me thinking, is that a physical certainty or could I be doing say 3,000 rpm in 5th at 121 km/h if I was going downhill or say, 3,500 rpm at 121 km/h if I was going up hill?
 

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:what:
 

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Yes for a manual or conventional automatic transmission, since the gears are fixed.

Not necessarily for a CVT (constantly variable transmission) since the gear ratios are not fixed and determined by the computer.

:)
 

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yes. the transmission and diff mechanically link the engine to the wheels. hence why you can over-rev - ie. 5th to 2nd (money shift). if the rpm could vary with speed in gear then the car wouldn't blow up as it would hold itself at the limiter. you'd be "mechanically over-revving" it.
 

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Yes for a manual* or conventional automatic transmission**, since the gears are fixed.
:)
*Assuming the car is not equipped with an electronic type overdrive that changes the final drive ratio on-the-fly, but even then, those are technically different "gears," and the answer is still yes.
And
**Assuming the torque converter is locked.

Minor nits, but this is TCL, so we gotta pick 'em.

:beer: :thumbup:
 

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The other night I was driving and I noticed that in 5th gear my car car's tachometer was at exactly 3,200 rpm. The speedometer was at 121 km/h. It got me thinking, is that a physical certainty or could I be doing say 3,000 rpm in 5th at 121 km/h if I was going downhill or say, 3,500 rpm at 121 km/h if I was going up hill?
With a manual transmission, or in an automatic with a locked-up torque converter, there's a direct mechanical connection between the engine and transmission. So your revs should be essentially the same regardless of load unless your clutch is slipping.

There's no physical connection from the engine to the transmission in a traditional automatic when the torque converter isn't locked up. That's why the revs initially start rising faster than the speedometer. In cars with tall gears like my Sonata, it can almost feel like the car has a CVT when it's getting moving.

Automatic cars will also unlock the torque converter when more power/revs are needed to, say, climb a hill. In some vehicles people mistake this for a downshift since there can be some shift shock and higher revs.
 

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No. Tire pressure and wear. Wear on a variety of gears. Tire spin or slide. Slipping clutch. Not to mention any number of sensor glitches.

That's not related to your problem though, or maybe it is.
 

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Will an engine always rev at the same RPM for a given speed in the same gear?
Manual transmisison?

The short answer is: Yes.

The long answer: Yeeeeeesssssssssssssssssssssssssssssssssssssss.

If the engine rpm does change in the same gear and at the same speed when experiencing an increased load (hill)- then you need a clutch.
 

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Discussion Starter #14
So engine load wont play into this in a manual car?
That's what I was wondering, but apparently not (unless the clutch is slipping).
 

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That's what I was wondering, but apparently not (unless the clutch is slipping).
The manual transmission is a solid mechanical link (with clutch engaged) between the engine and drive wheels. The only way the rpm could change at the same speed and gear would be if something in the mechanicals was slipping.

Now the traditional automatic has a torque converter- which is just a hydraulic pump.
And they do allow the engine rpm to change based on the load- but moderns ones also have a "lock up" feature that locks the torque converter (no slipping) for better fuel economy.


You might try howstuffworks.com for some good explanations of all this tuff. :thumbup:
 

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That's what I was wondering, but apparently not (unless the clutch is slipping).
So if I am running a car with a trailer, and one without a trailer, both manual 5 speeds, both at say, 60MPH, I will be running both car in the same RPM?

That does'nt sound right........

example, I want to go up a 25 degree incline at 60 KPH


If I have a empty car, no trailer............ok, say I will be doing the 60KPH uphill at 3000 RPM in 3 gear.

Now, take the same car, with 4 passengers, and a 1000 pound trailer.....if I want to go uphill ate 60KPH I will need to be running like 4500 RPM in the same 3rd gear. same thing car be said, if I only rev it to 3000RPM now, I wont reach 60KPH......


does this make sense? help me......
 

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So if I am running a car with a trailer, and one without a trailer, both manual 5 speeds, both at say, 60MPH, I will be running both car in the same RPM?

That does'nt sound right........

example, I want to go up a 25 degree incline at 60 KPH


If I have a empty car, no trailer............ok, say I will be doing the 60KPH uphill at 3000 RPM in 3 gear.

Now, take the same car, with 4 passengers, and a 1000 pound trailer.....if I want to go uphill ate 60KPH I will need to be running like 4500 RPM in the same 3rd gear. same thing car be said, if I only rev it to 3000RPM now, I wont reach 60KPH......
You probably will need more revs to maintain speed up that hill, but you won't find them in 3rd. Just flooring it won't arbitrarily increase the revs -- the car will just start slowing and bog down and, eventually, stall. You'll have to downshift to 2nd to get the revs up and maintain speed.

But, yes, at 60 KPH in 3rd gear, both vehicles would be at the same 3000 RPM.

If the car's 3rd gear had a shorter ratio that had the engine at 4500 RPM at 60 KPH then you would probably be okay. But the unladen car would also be at an identical 4500 RPM.


(This assumes we're talking about manual transmissions. A car with an automatic will just kick down to a lower gear as necessary based on load and throttle input.)
 
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