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Old 03-27-2021, 11:59 AM   #1
bucket
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Need speedo / gear help

I have a 79 K10 with 1/2 life 33 BFG AT KOs. 350/350/NP203. The 203 has been converted to part time and it has a 1:3000 ratio adapter on the speedo cable. The speedo reads little less than 10% fast (75 @ GPS 70). On the highway the odometer measures almost exactly 9 miles over 10 miles per mile markers. I have not pulled covers on the axles but I'm guessing they have been changed. My RPMs are about 2400 at GPS 70. I think they would be lower than this if the original 3.07 gears were in there. What gearing do you guys think I have? Is there another ratio adapter box that would get my speedo / odo more accurate?
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Old 03-27-2021, 03:58 PM   #2
57taskforce
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Re: Need speedo / gear help

Well, using the calculator below, it looks like with 3:07s you’d be running approximately 76-77 mph at 2400 thru 1:1 high gear.
With 3:42’s you’d be closer to 69-70. This is all estimations though because the actual tire size is going to change the results. That’s using a 33” height but at half tread your not likely going to be as accurate. Looks like its possible it may have 3:42, but I’d bet you’ve still got 3.07’s and here’s why, torque converter slip in that th350. If you had a manual transmission I’d say 3:42’s are likely, but with the auto it’s normal to run a few hundred Rpms higher than a manual transmission with the same gearing setup.

https://spicerparts.com/calculators/...rpm-calculator
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Old 03-28-2021, 11:36 AM   #3
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Re: Need speedo / gear help

Thanks! I had not considered torque slip. The truck isn't as big of a dog off the line as i thought it would be if it had 3.07s. Don't get me wrong. Its not real quick.
Ill pull the covers to confirm. Any ideas on a ratio box to get speedo more accurate?

[QUOTE=57taskforce;8900179]Well, using the calculator below, it looks like with 3:07s you’d be running approximately 76-77 mph at 2400 thru 1:1 high gear.
With 3:42’s you’d be closer to 69-70. This is all estimations though because the actual tire size is going to change the results. That’s using a 33” height but at half tread your not likely going to be as accurate. Looks like its possible it may have 3:42, but I’d bet you’ve still got 3.07’s and here’s why, torque converter slip in that th350. If you had a manual transmission I’d say 3:42’s are likely, but with the auto it’s normal to run a few hundred Rpms higher than a manual transmission with the same gearing setup.

https://spicerparts.com/calculators/...rpm-calculator[/QUOTE]
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Old 04-01-2021, 06:02 PM   #4
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Re: Need speedo / gear help

Quote:
The speedo reads little less than 10% fast (75 @ GPS 70). On the highway the odometer measures almost exactly 9 miles over 10 miles per mile markers.
Maybe I'm stupid or confused but that seems backward to me.
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Old 04-02-2021, 08:12 AM   #5
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Re: Need speedo / gear help

It is backwards from what I would expect too. There must be separate issues. I'm thinking the ratio box is wrong and the speedo head may need rebuilt or replaced.

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Originally Posted by 1976gmc20 View Post
Maybe I'm stupid or confused but that seems backward to me.
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