What RPM on tach=what speed

gokart25 · Nov 3, 2011

I just got a tach put on my bike (80cc).
I have a speedometer on it too.
I would like to get some input from everyone..
I am trying to see if the tach is reading the right RPM's.
What is a good benchmark. (25MPH=5000PRM)

My front sprocket is what came stock in the kit, and my back sprocket is 40tooth.

richirich · Nov 3, 2011

If you got to - MotoredBikes.com: Motorized Bicycle Forum - Powered by vBulletin the other site and go to the heading "wild in the streets" the first sticky should be a gear ratio calculator.
First enter 20 for the first gear(the small clutch gear) then 82 (for the large clutch gear) then 10 (for the small sprocket) then 40 for your rear sprocket, then input your tire size and rpm and it will give you your MPH

gokart25 · Nov 3, 2011

Very Cool thanks

thegnu · Nov 3, 2011

could you explain how you got that tach working on your bike ,I have been wanting to add 1 to my build an from what I have found here on the subject has done nothing but confuse me .
Gary

gokart25 · Nov 4, 2011

First of all I haven't full tested the one I bought.. just up and back on the street last night. I plan to ride today to do some more testing though.
I bought it for $20 off Amazon.
I made the selection of 1 fire per revolution, then wrapped the censor around the spark plug wire about 4 times and zip tied it there.
If you want.. I could get some pictures.

thegnu · Nov 4, 2011

that would be fantastic , thanks
Gary

biknut · Nov 4, 2011

gokart25 said:
I just got a tach put on my bike (80cc).
I have a speedometer on it too.
I would like to get some input from everyone..
I am trying to see if the tach is reading the right RPM's.
What is a good benchmark. (25MPH=5000PRM)

My front sprocket is what came stock in the kit, and my back sprocket is 40tooth.

20....crank gear
82....clutch gear

10....counter shaft sprocket
40....rear drive sprocket

rear wheel circumference
81.8"

RPM
1000 4.7
1200 5.7
1400 6.6
1600 7.6
1800 8.5
2000 9.4
2200 10.4
2400 11.3
2600 12.3
2800 13.2
3000 14.2
3200 15.1
3400 16.1
3600 17.0
3800 17.9
4000 18.9
4200 19.8
4400 20.8
4600 21.7
4800 22.7
5000 23.6
5200 24.6
5400 25.5
5600 26.5
5800 27.4
6000 28.3
6200 29.3
6400 30.2
6600 31.2
6800 32.1
7000 33.1

gokart25 · Nov 4, 2011

Thanks RichiRich and Biknut those are so cool, and right on the money.

scotto- · Nov 4, 2011

Well yesterday I was turning 6,310rpm on my 212cc geared at 10.80:1 and a CVT and was going 56.1mph. That's all I can tell you. Not sure what the final drive ratio is with the CVT but I was flyin' down the road for sure.....it'll do over 60mph with this gearing.

dnut

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What RPM on tach=what speed

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