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?

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

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]

Re: Need speedo / gear help
 

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

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.