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Old 09-28-2021, 04:41 PM   #1
Shoovel
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Default RPM check

Folks,
Just wondering what RPM everyone is turning at 50mph.

My 39 is turning 1500 to 1600RPM at approx 50 mph. I donít 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. 🤔
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Old 09-28-2021, 07:22 PM   #2
Zeke3
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Default Re: RPM check

Sounds like a pretty tall, low ratio, rear end. What size rear tire are you using?
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Old 09-28-2021, 07:36 PM   #3
tubman
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Default 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".
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Old 09-29-2021, 07:25 AM   #4
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Default 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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Old 09-29-2021, 10:12 AM   #5
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Default Re: RPM check

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You can back figure using the RPM to determine the rear axle ratio. All you need to know is the speed(MPH) and tire diameter. This is normally calculated in 3rd gear, so that is 1:1 and not a factor. If you have an overdrive engaged you do need to know the ratio and include it.

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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