Seems they are all mechanical and come from the Factory Inaccurate as mine is also. I pay no attention to it anymore. Its the nature of the beast... In all practicality there really nothing can be done. As far as the Tach I run a Trail tech tach, just a inductive wire rapped around the Spark plug wire and is spot on.
I was considering on doing something like that for a tachometer.
I know I could configure once I pickup speeds, 8,000 rpms equal 35 mph etc. Of course this varies upon roller weighs etc.
6,000 rpms with 3 - 4 gram rollers could be 15 mph.
6,000 rpms with 6 - 8 gram rollers 'could' be 25 mph or lower. All depending on the riders weight etc etc.
*just something to fiddle with.
As for faults, there's a possibility of a bad gear puck "I think that's what it's called".
I had a guy tell me in the past the cable becomes stretched and weak, causing it to become sensitive. I think it's bs.
I also read that there's a plate inside the cluster that can be adjusted by bending it..
I've had my eyes open for a dirt cheap replacement " under 20", but nothing so far.
I've also considered getting a digital speedometer, something you might buy to make a dirt bike street legal. Haven't found anything cheap under 100 yet. And I prefer not investing money like that in this crap bike >.<.
Do you oddly have a amazon link to the one you bought?
If you have a smart phone, there are motorcycle handlebar mounts available. Snap your phone in it, turn on your speedo app, and hit the road. Biggest drawback would be removing and re-installing at every stop you make.
The link I posted above will take you to Walmart......but these are available all over....bike shops, Amazon, Ebay, etc. Just measure your front tire's circumference, and then make sure the unit you buy can be set to that number. Example: My 110/90-12 tire covers 61.6" in one revolution. When I had a 120/70-12 tire on it, the measurement was 58.4 inches. (going from memory, numbers may be off by a couple of tenths)
Do a little research, you find one that'll work.
2011 Jonway 150cc...18,500 miles and counting
Other current rides: 1999 BMW, R1200C "Dooley" 2000 BMW, R1200C "Schultz" 2006 Ural Troyka "Boris"