Missing drive gear - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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post #1 of 10 (permalink) Old Feb 19th, 20, 2:56 PM Thread Starter
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Missing drive gear

Ok yíall, I am installing a sb350 and th350 in my 65 Chevelle and the transmission I got was missing the entire speedometer housing and drive gear. It is a BOP TH350. I understand there are speedometer calculators available online but the ones I have found are all based on the drive gear. Where do I start? Iím running 255/40/18 in the rear and have 3.36 stock rear end.

Thanks in advance.

Jacob
1965 Chevelle Malibu SS
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post #2 of 10 (permalink) Old Feb 19th, 20, 6:56 PM
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Re: Missing drive gear

transmissioncenter.net/shop/speedometer-gear-calculator

They also sell the gears.

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post #3 of 10 (permalink) Old Feb 19th, 20, 9:37 PM Thread Starter
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Re: Missing drive gear

Thank you, but again they are requiring a drive gear to calculate the speedometer gear, I donít have a drive gear.

Jacob
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post #4 of 10 (permalink) Old Feb 19th, 20, 9:53 PM
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So, pick one.
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post #5 of 10 (permalink) Old Feb 19th, 20, 10:22 PM
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Re: Missing drive gear

Gear ratio X 20.2 (static factor) divided by the tire diameter equals the speedo ratio.
3.36:1 rear axle ratio and a 26.0" tire will give you 2.610.

Drive gears for a T350 are in the range of 7-10 and driven gears are 17-25. Take the driven gear over the drive gear which gives you a ratio. The correct ratio will be the closest number to 2.610 in your case.

So take the 2.610 ratio and multiply it times the various driven gear ratios, then round to the closest whole integer, like below.

2.61 x 7=18.27/18=1.015
2.61 x 8= 20.88/21=99.43
2.61 x 9= 23.41/23=1.018
2.61 x 10=26.10/26=1.001

The closest combination to 1 is a 10/26 ratio. But the 26 tooth gear is not available.
The next closest is the 7/18 combination.
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post #6 of 10 (permalink) Old Feb 20th, 20, 12:02 PM Thread Starter
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Re: Missing drive gear

Awesome information, thank you. It appears that the drive/driven gears for my non bullet style GM/BOP th350 are quite different in the number of teeth, same math apply?

Jacob
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post #7 of 10 (permalink) Old Feb 20th, 20, 12:25 PM Thread Starter
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Re: Missing drive gear

Ok so hereís what it got -

2.61 x 15 = 39.15/39 = 1.0038
2.61 x 17 = 44.37/44 = 1.0084
2.61 x 18 = 46.98/47 = 0.9995
2.61 x 19 = 49.59/50 = 0.9918

Looking at eBay it doesnít appear that 47 or 50 exist so my best case is 15/39?

Jacob
1965 Chevelle Malibu SS

Last edited by Jakedub79; Feb 20th, 20 at 12:47 PM.
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post #8 of 10 (permalink) Old Feb 20th, 20, 1:09 PM
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Re: Missing drive gear

I hope you know that you're also going to need an adapter for the BOP trans to Chevy engine as they don't match up.

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post #9 of 10 (permalink) Old Feb 24th, 20, 10:30 AM Thread Starter
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Re: Missing drive gear

Adaptor to the block or to the driveline? Itís all installed currently, without one to my knowledge.

Jacob
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post #10 of 10 (permalink) Old Feb 24th, 20, 12:45 PM
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Re: Missing drive gear

Quote:
Originally Posted by Jakedub79 View Post
Adaptor to the block or to the driveline? Itís all installed currently, without one to my knowledge.
To the block. Post a pic of what you have.

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