Oh I just noticed i forgot to tell you about when to shift. As you said you where shifting at the blue marker.
For this it helps to know the power curve of the particular truck you are driving. Now i don't know the figures of the game, how it's calculated etc so we will just take some real world data to help explain how it sorta works in game.
But before we go further we have to understand the difference between torque and horsepower as well, in the most simple terms...
Horsepower helps you accelerate, it is derived from the torque of the engine but at higher RPM you always have higher horsepower. Thus it is easily said that to accelerate faster you should go higher in the RPM. But this isn't necessarily true, which brings us to...
Torque helps you do work, if you try and screw in a bolt using your fingertips it isn't going to become very tight. Easily loosened with any tool. You might have a billion horsepower at your fingertips but without a lever to help you screw in that bolt it will translate to nothing in terms of how tight that bolt is.
Likewise, if your vehicle is light then lots of horsepower will help you reach high speeds. But if your vehicle weighs a lot then you will need a lever to make any effective work on that vehicle. (or bolt)
Ok, so how does a bolt compare to a heavy vehicle? Well lets take another example, twist a pencil between your fingers... Easy right? The pencil doesn't weigh anything after all.
Now, take a car wheel and try to spin it at it's center the same way you did with your pencil... It will rotate but it takes a lot of work to get it up to speed.
BUT, if you now grab one of the bolts of that wheel (further out from the center) and yank it it will start to rotate much easier. You applied the same "horsepower" but you now have much more torque as well.
And finally, if you put your hand on the outside of the wheel you will make it spin with even less effort.
Now then... Speed.
Imagine if you could make your hand rotate around your wrist, say you could make it spin a full revolution every second. Putting your hand on the outside of the wheel will make it easy to spin the wheel but it won't go very fast. Now, if you could do the same at the location of the bolts on the wheel then you could get the wheel to spin faster but it will require more work to make it do so.
And finally, putting your spinning hand at the center of the wheel would make the wheel spin as fast as your hand but just like before, it would take a lot of work to get it up to that speed.
That's why we have gears, you start off on the outside of the wheel and gradually move closer to center with each gear change to keep making that wheel spin faster and faster.
Ok, so let's look at those power curves shall we?
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Source:
Cableprice Scania 16-liter engines.
Lets start off with the "small" (top picture) engine, when you initially take off you have very little power (horses) but you have lots of torque allowing you to get that heavy trailer behind you moving. As you rev up you notice that the vehicle accelerates faster and faster up til around 1400 RPM, where your effective torque starts to die off. Naturally this is also where the increase in horsepower starts to die off.
The reason for this is due to engine design, i will not delve deeper into that subject except to mention that the engine designers have made sure that the engine will deliver as much torque as possible at a certain RPM range for that engine. The stresses on the engine is the lowest at 1000 RPM but it still can supply as much work as it would at 1400 RPM. It just cannot accelerate as well at 1000 RPM.
As you should notice the amount of horsepower you gain after 1400 RPM is minimal, thus it's best to change to a higher gear at that point. (The green area on your tachometer reflects the same power ranges as in these pictures by the way, change gears when you are outside the green area!)
Now, lets take a look at the "bigger" (bottom picture) engine. Immediately there are 2 things you should notice here.
First is the optimal power range is shorter. (starts dropping at 1350 RPM)
Secondly is that the amount of power after this optimal range still rises pretty rapidly.
In game i can tell this works as expected, however i feel all trucks have rapid power gain after the optimal torque range. Thus when to change gears becomes slightly more complicated than just changing when you are outside the green area on your tachometer.
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So in conclusion, when do i change gears? I have found that it's best to change to a higher gear at around 1500-1600 RPM. This is where the torque is no longer strong enough to work through the weight and overall air/road resistance but i have gotten as much out of the horsepower (rate of acceleration) as i possibly can.
So why did i waste your time explaining the above if all i could have said was "change gears at 1500-1600 RPM"?
Because it depends... With a lighter load you don't need as much torque as you do with a heavy load. The same way your pencil spins really easily, a bicycle tire spins easier than a car tire etc etc.
Finding the optimal ranges to shift will become second nature if that's what you are after, to get the best acceleration with light loads (or no load at all) then shift up later and with heavier loads you really want to make use of all that torque. Or you can stay in the green area all the time because that will work just fine too!
EDIT:
Oh by the way, you might wonder what the blue marker is for?
Well, with the exhaust brake. The higher the RPM the more effective it is.
The blue marker is the upper limit (and most effective point) to use the exhaust brake.
On Volvo's that blue is a tapered line going down to about 1400 RPM. The idea then is to downshift to stay somewhere in the middle of this range (1400-2000 RPM) as much as possible while exhaust braking.
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Now to answer your last question, are there times where i never get to use 11th or 12th gear? Well yeah, if the road is twisty and the AI is slowing me down then i can do a whole delivery without ever getting to 11th or 12th gear. But as far as weight goes, it doesn't affect me as i am running the top engines for the trucks i drive. (Scania 730, Volvo 750 etc)
I bet it does matter of the lower powered trucks though. I will test this out at some point, possibly even making a let's play where i start over and drive the lesser trucks.
On extended hiatus.