1981 GPZ 550 Mechanical Tach

I have nearly finished the project and have been enjoying a few rides so far.
I know these bikes have the mechanical drive tachometers, but mine tends to jump from time to time.
Because of this I am not sure of my engine rpms and want to check on this.
I have oiled the tachometer cable but it is still jumps occasionally.
Can anyone advise me on what the tachometer should be reading at 75 and 80 mph in 6th gear.
I just don't want to be running at too high of rpms on the highway.

I appreciate reading the information given by all of the members and found many useful posts throughout my project.

Sorry, I don't know the answer, but I do know that these old bikes love to rev. As long as the gearing is stock you don't have anything to worry about. I'd count the sprocket teeth and compare to the factory manual for your bike. You can download the FSM here in the Articles tab of the forum if you don't already have it.

Assuming the cable is good, you could try lubricating the tachometer. a small amount in the tach cable orifice. I'd guess sewing machine oil would be good or some other light/fine lubricant. The tach gears are probably plastic, so that's a consideration.

5849 and 6239 RPM respectively, assuming the final gearing 16 tooth front and 38 tooth rear, and the tire has a circumference of about 80.4 inches.

Your magnet may be hitting the drum inside the tach.
Just in case you feel like taking the tach apart.
Mechanical speedometers work the same way.

Sorry about the delay in response, just been busy at work. Thank you for the input on this issue.
I will take the cover off this weekend to see what the front sprocket is but I know the rear is a 38 tooth.
What would the rpm difference between a 16 tooth sprocket and a 17 tooth front sprocket?
I did lift the bike up on a stand and taped a string in the center of the tread and came up with a circumference of 82 inches.
I will also try oiling the tachometer again this weekend.
 

my mistake. front wheel stuff not needed.

I was able to check the sprockets tonight and found the front is a 17 tooth and the rear is 38.
would the 17 tooth cause it to run at higher rpms at speed than if it was the 16 tooth?
 

No, opposite.

www.gearingcommander.com/

Thank you all for the advice. I took the tach apart cleaned and oiled it.
I had done this several years ago when i was slowly going through everything and it probably got dirty from just sitting up.
Now the rpms are more in line with what I remember from 1981.
I am hoping to finish the last section of exhaust and then I believe the project will be complete.

This is the first bike I have done and there have been some challenges, but I would not have changed it for the world.
Well maybe a few of them.
But buying this one on August 15, 1981 and putting it back on the road surely is not one of them