Tom Urani

I have replaced my 4+3 transmission with a 6 speed, resulting in the speedometer understating my speed by about 15%.

The plastic gear on my speed sensor has 43 teeth. I believe it should be replaced by one that has 15% fewer teeth (about 37).

Is my thought process valid and if so, where do I find this part?

Thanks.

toptechx6

Local Chevy dealer, should be PN 25513044 also available from many forum vendors such as GM Parts Direct, Gm Parts House, Eckler's etc.

rodj

Did you change your diff ratio also?

WVZR-1

The 15% less DOESN'T work! The accuracy of the speedo is the result of the proper drive/driven for a given axle ratio and tire size. I don't recall for starters a 43 tooth for the 4 + 3 application. Maybe that was part of an axle ratio/tire size change?

It's much easaier to start from "scratch" with a single "known" number and that is the "drive" gear (internal to the trans). Depending on tire size and axle ratio you can determine if it will be a simple caalibration from a "driven" with the correct sensor.

You don't mention which 6-speed? To help you there needs to be "no assumptions"! Which is it? Specific model if other than a ZF S6-40. The "drive" on the ZF is a known and the math can be completed with just a tire size and axle ratio. If it's other than a ZF the "drive" number needs to be a known!!

What is the trans, the axle ratio and the tire?

Tom Urani

Call me simple but why reinvent the wheel when the degree of error is all that is required to affect a solution?

toptechx6

WVZR-1's advice is sound, do the math or use this calculator that helps math challenged types like me get close, then buy the right gear.

http://ncane.com/l90

WVZR-1

You're "SIMPLE"!

Your original comment:

I was "attempting" to remove the "guess" or as you call it "about" from your situation and offer a "fix"! See the "SIMPLE" suggestion in the next post! It's as "quality a guess" I can offer "about" your correction!

WVZR-1

Here's a "simple" suggestion!

Assuming that the pair of gears and the sensor are a "matched set" you can go "no lower" than 40 tooth without causing a problem with the "mesh" of the drive/driven UNLESS you replace the "speed sensor"!

Buy a 40 tooth and check the calibration using a GPS or measured miles! If your math or your "GUESS" is in error you'll still have an error and be back!

Tom Urani

Good answer. Since I can't buy less than 40 teeth (7% correction) the solution can only be a new speed sensor or "electronic box" approach.

Thanks.

WVZR-1

A 40 tooth gear will amount to something near 15%. In MPH something near this: 60 MPH X 085 = 51 MPH. Since you can buy this for maybe $10 or less it's an "inexpensive way to see where you actually are"!!! Here's the deal "ole country boys (me included) "used to" say better than 2.5 MPH per tooth but less than or near 3.

There you are! It's SIMPLE! Spend $10 and "prove me wrong"!! You "GUESSED" at "THE ABOUT" so you might as well keep "GUESSING" at the "FIX"!

Tom Urani

Thanks everyone. I should have been more careful with my speech, the error was 14.63% based upon 10 measured miles versus odometer reading. I corrected the problem by purchasing an Electronic Ratio Adjuster (ERA) by Abbott out of Pine Bluff Arkansas. It was a quick and complete fix. This is a black box with 12 small switches that amount to a binary coding of (I suspect) the resistence within the box. The various switches are set either off and on according to your error, in my case I used 15%. The box has 4 wires; 2 go to the spped sensor the others are power and ground.

Thanks again,

TU

WVZR-1

Abbott is a quality product and they've been around for quite some time. If you had a first time successful install with an Abbott you'll likely have "trouble free" service. Where did you locate "the box" on the car?

I would certainly have liked you to have done the 40 tooth install though!

Are you "game" for some "simple math"?

Post your rear axle ratio/tire size/trans type/rolling radius and what you expect the drive tooth # in the transmission to be and let's try the math! You would have to measure the "rolling radius" (center of axle to ground). We could use "advertised" diameter but that usually results in an error of varying results!