[INJUNTOM]

3.50 gears, p275/60-15 tires

TH400 trans

currently has 18T drive gear and 42T driven gear

[screamingchief]

Cant get there with the 18 tooth drive gear...

17 tooth drive gear and 42 -or- 43 tooth driven gear should do the trick.

http://www.tciauto.com/tc/speedometer-gear-calculator

HTH

Bret P.

[INJUNTOM]

Thanks a ton. Switching trans and converter combos around and want to get the speedo right for a change.

[zeebo]

just an fyi..i have been through this before and so has rocky rotella (who posts here) he did an article for hpp a little bit ago and came to the conclusion that if the speedo calc. says for you to use say a 37 tooth driven gear; try a 38 tooth one, and you will be much closer to being accurate. basically +1 tooth from what is recommended. hope that helps..and, if you have a GPS its a great way to check speedo accuracy.

according to that chart, if you go with a 16 tooth drive gear, a 41 tooth will probably get you almost right on the money...the calc comes out to 40.29 teeth driven.

[Verdoro 68]

Here's the article zeebo mentioned. It's pretty helpful.

http://www.highperformancepontiac.co...eter_accuracy/

[Rocky Rotella]

Originally Posted by zeebo

just an fyi..i have been through this before and so has rocky rotella (who posts here) he did an article for hpp a little bit ago and came to the conclusion that if the speedo calc. says for you to use say a 37 tooth driven gear; try a 38 tooth one, and you will be much closer to being accurate. basically +1 tooth from what is recommended.

Thanks for noting the article guys!

I wanted to clarify that what we found wasn't necessarily the calculator's fault. It can only compute info based on input. If that input is faulty, well, then there's no way it can give you an accurate recommendation.

We have found the variance lies within the way the tire manufacturer's measure/state the overall diameter of its tires. BFG, for instance, states its 255/60/15 Radial T/A has an overall diameter of 27.1 inches. When you load the car on the tires and the sidewalls bulge out slightly, it reduces the overall tire diameter by 1/2 inch or so. Other tires may be different.

When we went back and tried calculating it with an accurately sized tire height for our application, the drive/driven gear recommendations came right in.

[zeebo]

i agree rocky..its the way the tire is measured for the calculation; ie: under no load--put the tire on the car, and the diameter decreases slightly, hence the need for the extra tooth on the driven gear to make the speedo more accurate.

[Peje]

Have a question: Gonna change from 3.55 to 2.56.Just to change the plastic gears? Even the metal gear?

[zeebo]

Originally Posted by Peje

Have a question: Gonna change from 3.55 to 2.56.Just to change the plastic gears? Even the metal gear?

do you know the tooth count on the metal drive gear? if so, you might not have to change that, just the driven gear. so you are going to replace a 3.55 with a 2.56, correct? and you know your tire diameter?