Need speedo / gear help

[bucket]

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?

[57taskforce]

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

[bucket]

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]

[1976gmc20]

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.

[bucket]

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.

Originally Posted by 1976gmc20

Maybe I'm stupid or confused but that seems backward to me.