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
R