What speedo gear do I need?

[fade94]

My speedo is showing 20 slower than I am going. I currently have a blue one in the trans just to see if it was the speedo or the gear. The gear that came out looked pink. Is this an actual gear color? Also, where do I get them?

[Palf70Step]

Try this link. It'll let you know what you need

http://www.tciauto.com/tech_info/speedo_gears.htm

If your a turbo350 tranny than a pink is 7 tooth and blue is 18 tooh. Big difference, so yes you need to go back a few teeth.

[Robert1970C20rstbukt]

Sounds like you need a gear with less teeth. Less teeth-spins faster, more teeth-spins slower.

I'm having the same problem but in reverse, the speedo in my truck reads about 10 to 15MPH fast. I figured that I need a 45T gear and a 40-45T housing.

[jimfulco]

www.novaresource.com/speedo.htm and ...speedo2.htm

There's a formula there for calculating the speedo gear ratio you need, and a list of available combinations you can use to get that ratio. You'll need your rear end ratio and your rear tire diameter. The TCI site mentioned above has a tire diameter calculator.

[pjmoreland]

What speed were you going when your speedometer was 20 MPH off?

You can buy replacement gears at a GM dealership.

[fade94]

Thanks for the info. Will have to figure out my gear ratio now. It's a 5 lug leaf rear.

[fade94]

Update. I believe the gear ratio is 3.07. I have 29" tall tires. The drive gear on the trans shaft is black, 8 teeth. The current driven gear is brown, 18 teeth. Do I need to go up or down on teeth count on the driven gear to get the speedo to show faster? Speedo is currently showing slower than I am going. I have used the TCI site above, but it shows to use what I have and it is off. Also, using the Nova website, it says I need to drop down to an 17 tooth driven gear but the TCI site does not show one listed. Any help is appreciated..

[Z28Owner]

Send it to me, and I'll send you mine its 20mph fast

[jimfulco]

See if your local Chevy dealer has a 17-tooth gear. If they can't find it, tell them to try part number 3987917.

[subtle71]

I just had to do this at the beginning of last week. In addition to the link that Bill/Palf70Step listed above, try this one:

http://www.bowtieoverdrives.com/index.shtml

I found this one helpful for calculating the info. The TCI site was helpful for the part numbers that I needed when it came time to order them from the Chevy dealership.

Dean

[farside847]

Hmmm, all the calculators want to know how many drive teeth I have now and I dont really want to take it apart until I have a new one to go in. Anyway to tell what I have now with a 3speed tranny and 3.73 gears? I am swaping to 3.08 gears with the same tranny. Tires are 29"

[jimfulco]

http://novaresource.org/speedo.htm has the formula and charts that you need.

Rear axle ratio x 20.2 / tire diameter = speedometer gear ratio
In your case, 3.73 x 20.2 / 29 = 2.598:1

Take this ratio, rounded to 2.60, to the charts. Since there's no gear combo that gives exactly 2.60, you'll have to use one that's as close as possible. If I need one that's about halfway between two available ratios, I'd prefer using the one that's a little less than what I need, because it will make the speedo read a little bit fast, instead of a little bit slow. My lead foot makes this a better choice .

Decide which gears you need, & buy them. A local GM dealer should have what you need.

[fade94]

Well, I did the "rotate tire and count drive shaft revolution". Now I'm really confused. The drive shaft rotates LESS than 2 times for ONE revolution of the tire. Wouldn't that make the ratio 2:?? Local Chev dealer does not have the 17 tooth gear in stock and can not find one locally.

[farside847]

Originally Posted by jimfulco

http://novaresource.org/speedo.htm has the formula and charts that you need.

Rear axle ratio x 20.2 / tire diameter = speedometer gear ratio
In your case, 3.73 x 20.2 / 29 = 2.598:1

Take this ratio, rounded to 2.60, to the charts. Since there's no gear combo that gives exactly 2.60, you'll have to use one that's as close as possible. If I need one that's about halfway between two available ratios, I'd prefer using the one that's a little less than what I need, because it will make the speedo read a little bit fast, instead of a little bit slow. My lead foot makes this a better choice .

Decide which gears you need, & buy them. A local GM dealer should have what you need.

Thanks! butbutbutbut. What is my trans gear? I dont know what chart to use

[jimfulco]

Might be able to tell by the color of the gear. Take out the driven gear (one bolt & a one-handed wiggle & pull maneuver), then see if you can look down the hole with a mirror & a flashlight, but be aware that the colors tend to fade with age.

Of course with the driven gear out, you could count its teeth, then see which drive gear matches it in the novaresource chart. That's assuming the speedo is correct now. Count the teeth more than once, because it's easy to lose count.

[fade94]

Ok. Got the 17 tooth from local dealer. Put it in. Now, it just jumps. The drive gear on the shaft looked fine. Anything else to check on this thing? Is there something special I needed to do to install the driven gear other than just put it in?

[jimfulco]

Is the speedo cable fully engaged in the small end of the gear? Sometimes you have to fiddle with it to get the square drive to engage.

[fade94]

I could have sworn it was all the way in, but I'll have to check. I may have pulled it out while hooking the other end back to the speedo.

[72str8flush]

Originally Posted by jimfulco

http://novaresource.org/speedo.htm has the formula and charts that you need.

Rear axle ratio x 20.2 / tire diameter = speedometer gear ratio
In your case, 3.73 x 20.2 / 29 = 2.598:1

Take this ratio, rounded to 2.60, to the charts. Since there's no gear combo that gives exactly 2.60, you'll have to use one that's as close as possible. If I need one that's about halfway between two available ratios, I'd prefer using the one that's a little less than what I need, because it will make the speedo read a little bit fast, instead of a little bit slow. My lead foot makes this a better choice .

Decide which gears you need, & buy them. A local GM dealer should have what you need.

What size gears are in a TH350? 1.5" or 3/4"? I also dont want to take mine out before putting new ones in.

[jimfulco]

Chevys usually have the 3/4" one, and Buick, Olds, & Pontiac usually have the 1-1/2" one. The housing that the gear rides in is also bigger on the BOP transmissions, and you can tell the difference from the outside. The small one is maybe an inch in diameter, and the big one is more like 2".

[tim_mc]

For anyone that might search this old thread. There's a convenient way to determine the correct speedo driven gear if your speedo is off. My speedo was reading 20% faster (72MPH) than actual (60MPH on GPS), or 12MPH faster. 12/60 = 0.20 or 20%. The rule of thumb is 1 tooth for every 5% of difference. In my case I had to add teeth to the driven (pencil) gear to slow down the speedo, 4 teeth since it's 20% too fast. My old driven gear was 20-tooth. I swapped it today for a 24-tooth (ACDelco P/N 3860347, yellow, 3/4"OD). The speedo is now accurate. Luckily I did not need to change the 7-tooth drive gear in the tailshaft. Truck has 235/55R17 tires and a 3.07 rear end.

[Hart_Rod]

Originally Posted by tim_mc

For anyone that might search this old thread. There's a convenient way to determine the correct speedo driven gear if your speedo is off. My speedo was reading 20% faster (72MPH) than actual (60MPH on GPS), or 12MPH faster. 12/60 = 0.20 or 20%. The rule of thumb is 1 tooth for every 5% of difference. In my case I had to add teeth to the driven (pencil) gear to slow down the speedo, 4 teeth since it's 20% too fast. My old driven gear was 20-tooth. I swapped it today for a 24-tooth (ACDelco P/N 3860347, yellow, 3/4"OD). The speedo is now accurate. Luckily I did not need to change the 7-tooth drive gear in the tailshaft. Truck has 235/55R17 tires and a 3.07 rear end.

I have to change the drive gear on the tailshaft. What all is involved besides pulling the driveshaft and the rear tail housing? Thanks!

[tim_mc]

Here's a good thread on pulling the tail housing and changing the drive gear:
http://67-72chevytrucks.com/vboard/s...d.php?t=462914

[Hart_Rod]

Originally Posted by tim_mc

Here's a good thread on pulling the tail housing and changing the drive gear:
http://67-72chevytrucks.com/vboard/s...d.php?t=462914

Thanks!

[Sheepdip]

Originally Posted by Robert1970C20rstbukt

Sounds like you need a gear with less teeth. Less teeth-spins faster, more teeth-spins slower.

I'm having the same problem but in reverse, the speedo in my truck reads about 10 to 15MPH fast. I figured that I need a 45T gear and a 40-45T housing.

Who in the hell looks at their speedo in reverse???