09-28-2021, 04:41 PM | #1 |
Member
Join Date: Mar 2021
Location: Milton Ontario Canada
Posts: 83
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RPM check
Folks,
Just wondering what RPM everyone is turning at 50mph. My 39 is turning 1500 to 1600RPM at approx 50 mph. I don’t know what the rear axle ratio is. I thought it seemed low, so I connected up my MAC tester tach and it reads the same as the tach I installed. 🤔 |
09-28-2021, 07:22 PM | #2 |
Senior Member
Join Date: Dec 2014
Location: Columbus, IN
Posts: 1,407
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Re: RPM check
Sounds like a pretty tall, low ratio, rear end. What size rear tire are you using?
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09-28-2021, 07:36 PM | #3 |
Senior Member
Join Date: Dec 2013
Location: Minnesota, Florida Keys
Posts: 10,319
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Re: RPM check
My '51 has 225/75R15 rear tires and a 3.55 rear end. I run 2700 rpm at 65 mph. A little math yields a bit over 2000 RPM at 50 mph. If I remember, I'll check what I actually run at 50 mph tomorrow.
I will agree with Zeke that it seems you are a little "over-geared". |
09-29-2021, 07:25 AM | #4 |
Senior Member
Join Date: May 2010
Location: Norfolk UK
Posts: 214
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Re: RPM check
I'm assuming your Columbia is engaged for those RPM figures. Be about right for 600 x 16 tyres and a 2.72 ratio which is overdrive on a 3.78 final drive.
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09-29-2021, 10:12 AM | #5 |
Member Emeritus
Join Date: Nov 2012
Location: Wichita KS
Posts: 16,132
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Re: RPM check
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mph = (rpm x tire diameter) / (gear ratio x 336) rpm = (mph x gear ratio x 336) / tire daimeter gear ratio = (rpm x tire diameter) / (mph x 336) tire diameter = (mph x gear ratio x 336) / rpm |
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