Speedo Tuning

[Chevy305]

So I have a 85 Chevy K10 with 35in tires and the speedo was close to acurate. Well recently I regeared the axels to 4.10 from a 3.08 and now the speedo is way off and says I'm going faster than what I really am. I guess the speedo was calibrated for 35s and 3.08 and not 35s and 4.10s. So how do I make the speedo accurate again? I know it has to do with the little gears on the tailshaft of the t-case. But what do I do to with them?

BTW it is mechanical speedometer with the full cable n' such.

 

[Blue85]

First you should pull the speedo gear out of you transfer case and see what color the drive gear is and what color the driven gear is. Then you want to adjust the number of teeth by the same ratio as the ratio of your gearing change. That is: #teeth on new driven gear = (# teeth on old driven gear)x(4.10/3.08). This assumes that the speedo was calibrated correctly before. The problem is that there may not be a driven gear with enough teeth, so you may have to swap the drive gear as well. That requires that you pull the tailshaft housing off the T-case, but it's not too bad.

Or you can figure out what you need from scratch. All speedometer cables are supposed to go at the same speed. I believe it's 1000 revolutions per mile. Make sure you use the actual loaded tire radius and not the diameter marked on the sidewall.

Here's some info on speedometer gearing: http://www.transmissioncenter.net/speedometer_calibration_______va.htm
http://www.tciauto.com/tech_info/speedo_gears.htm

I think I determined once that the color codes correspond to the same number of teeth whether the gear is mounted in the transmission or the transfer case. I don't remember if they are physically the same gear, though.

I have gotten these gears from a local transmission shop before. They had boxes full of used ones and they sell for a couple of dollars or for free if you're buying other parts.

 

[trailblazr81]

This was probly 5 years ago when I actually did this but I found a couple webpages that actually had the conversions (4.10 gears with 35" tires needs X teeth gear). Then I just went to NAPA and had them order a speedometer gear with X many teeth. Had it the next day.