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Old 09-20-2018, 09:07 PM   #1
oneperfectrun
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What rear gear do I have?

Any of you guys or gals know what rear gear ratio I might have? The truck is a 1970 C10, half ton, originally inline 6 and powerglide. 12 bolt rear.

I have a 327 and 700r4 in it now. Trans has been in for a few years. The trans didnít have a speedo gear on the shaft at install and I just got gears shipped in yesterday to get speedo going again. I put the gears in this afternoon and speedo is 9-10 mph slow according to gps.

The truck has P235/75R15 tires that measure right at 28Ē tall. With the ratio calculator I used I figured for 4.10 gears and came up with 15 tooth drive gear and 43 tooth driven gear.

SPID doesnít give the ratio that I can tell. I just assumed gears were 4.10.

Anyone know what gears were common for the inline and powerglide?
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Old 09-20-2018, 09:35 PM   #2
jocko
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Re: What rear gear do I have?

1970 C10 w/6+PG was 3.73 standard, 4.11 optional - so if you have nothing called out on the SPID, the truck was born with a 3.73. No guarantee that's what's STILL in it, but as that how it came into the world.
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Old 09-20-2018, 09:48 PM   #3
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Re: What rear gear do I have?

Quote:
Originally Posted by jocko View Post
1970 C10 w/6+PG was 3.73 standard, 4.11 optional - so if you have nothing called out on the SPID, the truck was born with a 3.73. No guarantee that's what's STILL in it, but as that how it came into the world.

Appreciate that info. Iím pretty confident that the original ring and pinion is in the truck. Iíll most likely pull the cover this weekend and count the teeth to confirm before ordering more speedo gears.

With the 700R4, converter locked up, it cruises along at 70 mph right around 2k rpm.
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Old 09-21-2018, 07:03 AM   #4
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Re: What rear gear do I have?

https://tiresize.com/comparison/
https://spicerparts.com/calculators/...rpm-calculator

I see your tire size is closer to 28.9"
In the transmission calculator I got:
Ring & Pinion: 3.5
Tire Height: 28.9
Engine RPM: 2000
Transmission Gear Ratio: 0.696

Vehicle Speed: 70.62

If I change that to 3.73, I get 66.26, so you would be over 2K RPM at 70 mph. Played with it a bit, ~2100 RPM at 70 mph.
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Old 09-21-2018, 08:09 AM   #5
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Re: What rear gear do I have?

Quote:
Originally Posted by rpmerf View Post
https://tiresize.com/comparison/
https://spicerparts.com/calculators/...rpm-calculator

I see your tire size is closer to 28.9"
In the transmission calculator I got:
Ring & Pinion: 3.5
Tire Height: 28.9
Engine RPM: 2000
Transmission Gear Ratio: 0.696

Vehicle Speed: 70.62

If I change that to 3.73, I get 66.26, so you would be over 2K RPM at 70 mph. Played with it a bit, ~2100 RPM at 70 mph.
I measured my tires on two separate occasions. Just a tick under 28Ē. They are mounted on factory 8Ē rally wheels so Iím thinking they may be just a bit shorter with the wider wheels. A friend has his truck at my shop and his has same size tires on the factory steel wheels. There is a visible difference with the two sitting side by side.
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Old 09-21-2018, 08:56 AM   #6
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Re: What rear gear do I have?

might be a difference in sidewall rigidity, or tire pressure. I think when the vehicle is in motion, the tires round out more as opposed to having a bit of a flat spot at the bottom.

looking at http://67-72chevytrucks.com/vboard/s...d.php?t=578793 and it look like the 3.36 ratio was available in 1970.

I tried 3.36 rear with 28" tires. 70 mph would be about 1950 RPM.
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Old 09-21-2018, 10:11 AM   #7
oneperfectrun
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Re: What rear gear do I have?

Think Iíll just pull the cover off this weekend and count the teeth. Im sure the gear oil needs changing anyway.
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Old 09-21-2018, 11:07 AM   #8
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Re: What rear gear do I have?

Quote:
Originally Posted by rpmerf View Post
looking at http://67-72chevytrucks.com/vboard/s...d.php?t=578793 and it look like the 3.36 ratio was available in 1970.
3.36 wouldn't have been an option for OP's config (I6/PG).
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Old 09-21-2018, 02:26 PM   #9
71CHEVYSHORTBED402
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Re: What rear gear do I have?

Quote:
Originally Posted by jocko View Post
1970 C10 w/6+PG was 3.73 standard, 4.11 optional - so if you have nothing called out on the SPID, the truck was born with a 3.73. No guarantee that's what's STILL in it, but as that how it came into the world.
Learn something new everyday. Mine is 3.73 as well.
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