Speedometer pinion gear formula

[Dino]

Does anyone know how to calculate the exact pinion gear needed?  I bought a 28 tooth gear pinion following one of the charts.  I used 2.94 gears and a 28" tall tire.  It was only later I realized my gears are 2.96 and the tire, according to the manufacturer, is 27.9" tall.  Do I need a new pinion?

[A383Wing]

there is a chart somewhere...but I have heard it's not all that accurate...what are you running that has 2.96 gears? I have never heard of that ratio...2.94 I have heard of, but not 2.96

Does car run? Got a GPS??

One tooth on the gears is about 5MPH if I remember

pic of a chart below

Bryan

[Dino]

The 2.96 gears came out of a '65 Coronet.  I thought it was strange as well but I saw the number stamped on the gear.  With GPS showing 55, my speedometer reads 45.  I have no idea what pinion gear is in there currently.

[A383Wing]

28 should be about right..it's never gonna be perfect...you will need to find out what gear is in there now...then subtract 2 teeth to get close...

Bryan

[oldcarnut]

One tooth on the gears is about 5MPH if I remember
pic of a chart below
Bryan

I was thinking it was close to 7mph-1 but I'm not positive I remember remembering that or that's what it ended up being for me with figuring one out way too many moons ago.  I do remember though the faster the speed the greater the difference so you may want to consider where your most time driving speed is at.

[A383Wing]

The MPH difference will vary with speed..I was quoting the difference from around 60 MPH or so

Bryan

[XH29N0G]

28 should be about right..it's never gonna be perfect...you will need to find out what gear is in there now...then subtract 2 teeth to get close...

Bryan

a change of 0.1 on radius and 0.02 on rear ratio will make a difference much less than one tooth.

[John_Kunkel]

For any formula you need to know the actual diameter of the tire. Best way to do this is to paint the tread of the tire with a small daub of paint and then roll the car until two paint spots appear on the pavement, this is the circumference. Measure between the paint spots and divide that distance by 3.1416 to get the diameter. Use that diameter on any charts.

[A383Wing]

if the car does not roll, jack up car until tire is off ground. Take a tape measure and measure all the way around the center of the tread on tire. Take that and divide by 3.1415926 and that will be your diameter as well

Bryan

[Paul G]

The 2.96 gears came out of a '65 Coronet.  I thought it was strange as well but I saw the number stamped on the gear.  With GPS showing 55, my speedometer reads 45.  I have no idea what pinion gear is in there currently.

Has the speedo been calibrated? It works by magnetism. The magnet spins, the speedo needle is held at zero by a spring, magnetism between the spinning magnet and the metal housing of the needle is what determines speed. Very sloppy system.

Check your odometer against the mile markers on the highway. I would bet your odo is just about correct, 1 mile traveled on the highway will tick off 1 mile on your odo. You may have the most correct gear already with the 28 tooth. If the odo is correct your speedo may need calibration.

[Dino]

I measured around the wheel and did the calculations.  The 28 tooth gear should be pretty spot on so here's hoping.   

Paul, I'm not sure what the current pinion gear is, this car used to have an entirely different drivetrain originally.  When I had the smaller wheels and 3.55 gears the odometer and speedometer were fairly accurate.  Once I changed the gears to 2.96 everything was off again and now that I changed the wheels it only got worse. 

I didn't buy another gear when I changed the pumpkin gears because I knew I was going to buy different wheels.  Now that I did it's time to get it reading a bit more accurate. 

I will check the odometer against the mile markers and see if the speedometer needs calibrating.

[Paul G]

Re reading your original post, you have a 28 tooth gear just not installed yet, corect? I think you will be ok wih

[Dino]

Re reading your original post, you have a 28 tooth gear just not installed yet, corect? I think you will be ok wih

Correct, I bought the 28 tooth gear and then started wondering if I made the right choice.  We'll find out as soon as I find out how to replace it!

[maxwellwedge]

Make sure to "index" it properly in the case. 

[Dino]

Make sure to "index" it properly in the case. 

I just looked up how to do this and noticed you have to line up the gear with the correct number range on the case so I'll be sure to do that.   

[bull]

Make sure to "index" it properly in the case. 

I just looked up how to do this and noticed you have to line up the gear with the correct number range on the case so I'll be sure to do that.   

Do you have a link for this procedure?

[Dino]

Make sure to "index" it properly in the case. 

I just looked up how to do this and noticed you have to line up the gear with the correct number range on the case so I'll be sure to do that.   

Do you have a link for this procedure?

Sure do!

http://www.dodgecharger.com/forum/index.php/topic,74853.msg839457.html#msg839457

[bull]

Wow, Johnny on the spot. Thank you!

[cudaken]

Dino, I have a new one I have never installed. It is Mopar part # 1-52068152 and the gear is yellow.

See if someone can tell you if the parts number is correct for your needs.

           Ken