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Old 10-03-2012, 05:45 PM   #1
Yachtie OP
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Tach Ratio?

Might be doing a tach and speedo swap and was told before ordering to find out our tach ratio. Any ideas? My bike is a 76 r90/6
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Old 10-03-2012, 08:37 PM   #2
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Quote:
Originally Posted by Yachtie View Post
Might be doing a tach and speedo swap and was told before ordering to find out our tach ratio. Any ideas? My bike is a 76 r90/6

Here are final drive vs speedo ratios....not sure about "tach" ratios...

ratioring/pinion teethrpm @55mphrpm @70mphspeed @7200rpmspeedometer ratiousual model
2.9132/11305038811301.078R100
333/11314740051261.112R90S-R100
3.0934/11324141251221.144R90-R100/7
3.232/10335742731181.186R75-R80
3.3637/11352544861121.244R60-R75
3.4431/9360845921101.259
3.5632/9373447521061.318R60-R65
2.6234/13This was for racing, /5 and later
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Old 10-04-2012, 06:23 AM   #3
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Thanks. So here's another question, the speedo they carry that I wanted to go with has a ratio of 2240 rpms @ 60 MPH. Would I run into any significant issues with that? They carry tachs In a 1:4 1:5 1:7 an they think a 1:5 will work.



Quote:
Originally Posted by bmwhacker View Post
Here are final drive vs speedo ratios....not sure about "tach" ratios...

ratioring/pinion teethrpm @55mphrpm @70mphspeed @7200rpmspeedometer ratiousual model
2.9132/11305038811301.078R100
333/11314740051261.112R90S-R100
3.0934/11324141251221.144R90-R100/7
3.232/10335742731181.186R75-R80
3.3637/11352544861121.244R60-R75
3.4431/9360845921101.259
3.5632/9373447521061.318R60-R65
2.6234/13This was for racing, /5 and later
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Old 10-04-2012, 08:10 PM   #4
Wirespokes
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You'll need a speedo that reads 60 when the cable spins at 1,144 RPMs. That is, if you've got the stock ratio final drive.

The tachs are all 1:2. In other words, the tach spins half engine speed.
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Old 10-04-2012, 08:51 PM   #5
disston
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Since the tach is driven off the cam shaft which is turning one haft crank speed aren't the tachs spinning the same as cam speed? If the tach is spinning 1: 2 then it would be spinning 1: 4 to the crank or engine RPM.
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Old 10-05-2012, 08:09 AM   #6
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Good point, Charlie! The tach would spin 1/4 engine RPM.
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Old 10-05-2012, 09:17 AM   #7
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Originally Posted by Wirespokes View Post
Good point, Charlie! The tach would spin 1/4 engine RPM.
Great! SO 1:4 it is! Thanks so much guys!
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