A little speedo gear help needed please.

[1970pelle]

With a the changing around on my 86 C20 I have done. I thought I had it figured out when I ordered the new trans. Well I'm running a good 10 miles an hour fast, at 25 and 35. Then at 45 it spreads out a bit more. I have a 4X2 with a turbo 400, using a 44 tooth gear in a 40/45 housing.
I have here a 40T and a 41T gear, do you think these will help me get closer to my correct goal? I'm running the stock steel 16X7 wheels, and 215/85R16 tires on all four corners. I know more info is needed, just thought someone might know right off hand. Thanks Ken

[GRADYS Performance]

Go to TCIAUTO.COM they have a calculator in the technical resource section that will tell you exactly what gears you need. 3 of the following 4 are needed to figure out the fourth.

Tire size or Diameter

rear end gear ratio

drive gear tooth count

driven gear tooth count.

Drive gear is on the output shaft of the transmission. Count teeth thru speedo housing hole or take off tail housing to count.

[MikeB]

Originally Posted by 1970pelle

I'm running a good 10 miles an hour fast, at 25 and 35.

Hey Ken, are you saying the truck is actually moving approx 10 mph faster than indicated by the the speedo at 25-35 mph?

If that's the case, you'll need to make the driven gear spin 30%-40% faster. So if you currently have, for example, 10/44, you will need to go to somewhere around 14/40 if I did the math right, and that's a big if.

Also, I'd check actual vs. indicated speed at 30 mph and 60 mph and determine how far you're off percentage-wise. If it's 33% at one speed and 37% at the other, then try to get as close to a 35% ratio change as possible. Or if you do a lot of highway driving, then focus on the speedo being most accurate at 60-70 mph.

A good way to check actual road speed is with a free smart phone app.

[1970pelle]

Originally Posted by MikeB

Hey Ken, are you saying the truck is actually moving approx 10 mph faster than indicated by the the speedo at 25-35 mph?

If that's the case, you'll need to make the driven gear spin 30%-40% faster. So if you currently have, for example, 10/44, you will need to go to somewhere around 14/40 if I did the math right, and that's a big if.

Also, I'd check actual vs. indicated speed at 30 mph and 60 mph and determine how far you're off percentage-wise. If it's 33% at one speed and 37% at the other, then try to get as close to a 35% ratio change as possible. Or if you do a lot of highway driving, then focus on the speedo being most accurate at 60-70 mph.

A good way to check actual road speed is with a free smart phone app.

Thanks Mike. I had a Black and Yellow gear here at the house. The Black gear took care of my issue. I know I could fix it without going though all the jumping through hops and stuff. I simple calculator is all I need. Ken

[Marshy]

This is one issue that I need to resolve on my truck but just don't know where to begin. Can anyone point me to where the speedo gears are located on a '84 K10?

[83GMCK2500]

Originally Posted by Marshy

This is one issue that I need to resolve on my truck but just don't know where to begin. Can anyone point me to where the speedo gears are located on a '84 K10?

Transfer case output shaft and the cable drive. In cutaway pic the green and gray gears. Follow speedo cable back to transfer case and look for the drive (other pic) where it's clamped into the rear housing of the NP208.