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Old 22nd June 2004, 21:05   #1
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Could any of you please explain the practical relation between the
RPMs and the gear you are in / the speed you are driving at?

To rephrase:

Is there a certain RPM that needs to be maintained depending on which
gear the car is in or at what speed the car is travelling? What
difference does this make / impact does it have on day to day driving?

Thanks in advance.

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Old 22nd June 2004, 21:45   #2
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Hey Neverclever,

Well to ground you, first understand that horsepower is (torque x rpm)/5252. So it is safe to say that as you go up in the RPMs your engine puts out more and more power (until a certain limit).
That is why you raise the engine when starting from a standstill for example, because at idle ~850RPM the engine is not putting out enough power to easily move the car.

Now, the peak power of an engine usually is produced at the high end of the RPM spectrum, so if you are looking to overtake for example, you downshift to get the engine spinning faster and producing more power which enables you to overtake.

For everyday driving its possible to never go above 3500RPM, and i believe as a generalization most petrol engines give their best fuel efficency figures between 2000-3000RPM (am i right?? anyone?)

Now to answer ur question>
Quote:
Originally Posted by [b
Quote[/b] ]the practical relation between the RPMs and the gear you are in / the speed you are driving at?
Think of a gear ratio as a multiplier. (I am gonna use some hypothetical numbers in the example, as well as exclude the differential, aka final drive ratio...which is like a constant ratio of reduction no matter what gear you are in)

If your engine is spinning at 1000RPM and you are in 1st gear (which has a ratio of 0.20, ie the driveshaft spins 0.2 time every time the engine spins once) the driveshaft comming out of the gearbox will be spinning at (1000x0.20=) 200RPM.

Now if your engine is at 1000RPM and you are in 4th gear (which we shall assume is a 1:1 spin ratio ie 1.00) the driveshaft comming out of the gearbox will be spinning at (1000x1=) 1000RPM.

Now that you have that down, all you have to do to understand this is put a tire on the driveshaft comming out of the gearbox, say with a circumference of 1meter (so you travel 1meter every 1Revolution).

Using that as a constant you can figure out that at 1000RPM in first gear you are travelling 200meters per minute
but if you are at 1000RPM in 4th gear you will be travelling 1000meters per minute.

So this shows how engine RPMs are inversely proportional to how high a gear you are in, (assuming the speed remains constant)


BUT.... you cant drive in 4th gear all the time becoz when you are in 4th at lower speeds your engine RPMs will be so low that your engine will not be creating enough power as discussed earlier, and thats why you have to work up the gears, because that way you manage to keep the engine at RPMs which produce enough power to keep the car moving.

eg:
1st gear 3000RPM 20km/h
2nd gear 3000RPM 45km/h
3rd gear 3000RPM 70km/h
4th gear 3000RPM 110km/h

I hope you have understood somehting or the other from this post, and i havent made it unneccessarily confusing!
do let us know what we can further clarify for you!
cya
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Old 22nd June 2004, 22:01   #3
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good work Rehaan...i had a few doubts, which has cleared now..thanks man.
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Old 23rd June 2004, 03:31   #4
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thankx rehaan for the information
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Old 23rd June 2004, 05:15   #5
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Great job Rehann

Now, can you include Overdrive in to the scene.

My car has overdrive switch. If it is switched on, the car seems to be continuing in higher gear even at the increase in accleration (and better mileage...).

BTW, my car has automatic transmission.

But if the Overdrive is off, at the slightest increase in acclearation, the auto switches to lower gear. Poor milage but good when taking a ramp on to Interstate on my way to work. Take the ramp at 20mph and exit the ramp on to Interstate at 60 mph.

Now can you please explain the theory of Overdrive?

Thank you
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Old 23rd June 2004, 16:00   #6
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Hi Satish,

"overdrive" is the term used for any gear that actually has a ratio of 1.0 aka 1:1 or greater. ie the output shaft is spinning faster than the input shaft.

Like in my earlier example Now if your engine is at 1000RPM and you are in 4th gear (which we shall assume is a 1:1 spin ratio ie 1.00) the driveshaft comming out of the gearbox will be spinning at (1000x1=) 1000RPM. that would be overdrive, as well as 5th gear in that gearbox which would have a ratio of 1.11 for example.

The reason you get better FE (fuel economy) in overdrive on the highway is because it manages to keep the engine spinning at lower speeds while the car is travelling at a high speed. The power generated at lower engine speeds is good enough to keep the car mantaining the highway speed, but is not enough to speed it up from 20mph to 60mph to get onto the highway for example, and hence a downshift is required for that.

Another reason your FE is better in overdrive is because in most auto transmissions, engaging overdrive locks up the torque converter, which is basically like a "slipping clutch" in an AT. So locking this prevents the wastage of engine power at the torque converter.

And enabling overdrive on your AT sounds like it prevents the `box from downshifting, which keeps the engine at the same low rpms.

Hope that helped,
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