It's more of a feel thing. My bike makes a noticeable noise when I'm hitting the area to shift. Sounds as if the engine is trying too hard, which it is, and I know it's time to shift. Each bike is different. A good rule of thumb for my bike, 600 vlx, is 20-25 in first, 40-45 in second, 60-65, in 3rd, and 4th for the rest. Just gotta find the spots for your bike.
you shouldn't need a tach to know when to shift. you can feel and hear when you need to. you should know if you are revving it too much or lugging it. it'll just take time.. especially if your new to ridin'.
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shift by ear and by feel of when the engine wants to change gears, dude. Bikes have wide rpm ranges for a reason. You'll end up crashing yourself if you're staring at the speedometer overthinking when to shift. just my .02.
For me, I find that shifting a few 1000RPM before the rev limiter is better than shifting closer to the rev limiter. On my SV650 I shift at a very high RPM close to redline, where as the Shadow it seems to like being shifted sooner than if it were the SV650.
Papasmurf, don't speedos typically read faster than you're actually going? In which case would I shift when it says those speeds, or when actually going that speed?
Speedometer accuracy will vary per bike but shouldn't be so much you need to worry about it now. I've read that some Shadows are off by 10%, usually reading higher than actual. And I've also read about others that were a little lower than actual. If your bike has mods to the tires or wheels it's more likely to be off and if you're really concerned and have money to burn, you can install a speed corrector.
Those published shift speeds are just landmarks for people not used to shifting manually. I'm assuming you don't have any previous experience driving a manual transmission. DO NOT WATCH THE SPEEDOMETER TO DECIDE WHEN TO SHIFT!
At first, around those published speeds (a quick glance), just notice the sound of the engine straining and the decreasing change in acceleration per throttle movement. It won't take long to learn to sense when to shift. As Jammit is implying, don't overthink it. Chances are, you won't break anything (at least not right away ) and a motorcycle wet clutch is a lot more forgiving than a car clutch.