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Old 12-29-2018, 04:51 PM   #1
1929
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Default RPMs?

Took the car out for its weekly winter exercise, speed was 58 mph at 2400 rpms, is that about normal for a 3:54 in the rear?
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Old 12-29-2018, 06:59 PM   #2
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Default Re: RPMs?

Try this link on rpm, rear end ratio, tire diameter and speed. You can plug in your values and check the rpm or change any three values to get the fourth.
http://www.advanced-ev.com/Calculators/TireSize/
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Old 12-29-2018, 08:27 PM   #3
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Default Re: RPMs?

That is correct
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Old 12-29-2018, 08:29 PM   #4
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Default Re: RPMs?

It depends on wheel diameter. Assuming from your user name that it is a 1929 model.
If you have 21" wheels with a dia of approx 30" your speedometer should show close to 62 mph for a 3.54 rear.
If a 30-31 with 19" wheels, your speedometer would be closer to 60 mph with a 3.54 rear.

If you are reading 58 mph I would believe you have a standard 3.78 rear and not a 3.54. Why do you think you have a 3.54 rear? Did you rebuild and change ring & pinion?
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Old 12-29-2018, 08:32 PM   #5
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Default Re: RPMs?

Quote:
Originally Posted by 1929 View Post
Took the car out for its weekly winter exercise, speed was 58 mph at 2400 rpms, is that about normal for a 3:54 in the rear?
Yes, that's about right.
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Old 12-30-2018, 10:41 AM   #6
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Default Re: RPMs?

Quote:
Originally Posted by GRutter View Post
It depends on wheel diameter. Assuming from your user name that it is a 1929 model.
If you have 21" wheels with a dia of approx 30" your speedometer should show close to 62 mph for a 3.54 rear.
If a 30-31 with 19" wheels, your speedometer would be closer to 60 mph with a 3.54 rear.

If you are reading 58 mph I would believe you have a standard 3.78 rear and not a 3.54. Why do you think you have a 3.54 rear? Did you rebuild and change ring & pinion?
I installed the 3:54, but, I have 16 inch wheels.
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Old 12-30-2018, 11:04 AM   #7
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Default Re: RPMs?

That makes more sense.
My chart shows a rolling circumference for a "typical" 16" tire to be 89.10", which would put you at 57.1 MPH at 2400 RPM with a 3.54 rear. With the variation in tires, you could be spot on at 58
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Old 12-30-2018, 12:17 PM   #8
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Default Re: RPMs?

Quote:
Originally Posted by GRutter View Post
That makes more sense.
My chart shows a rolling circumference for a "typical" 16" tire to be 89.10", which would put you at 57.1 MPH at 2400 RPM with a 3.54 rear. With the variation in tires, you could be spot on at 58
Thanks GRutter
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