Speedometer Gear Reducer

[69 Post Sedan]

I changed the gears in the Chevelle from 3.73 to 4.56 with 28" tall tires. I need a speedometer gear reducer. The drive gear has 8 teeth and the driven gear has 21 teeth.

Does anyone know how to figure out which one I need or have a part number. The little research I have done says I need one that's has a 20% reduction? <<GRAEMLIN_URL>>/dunno.gif

Thanks, Kurt

[Postsedan]

Did you call Crash?

Dan

[69 Post Sedan]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Postsedan</div><div class="ubbcode-body">Did you call Crash?

Dan </div></div>

Not yet but the guy who built my trans was really close as I need about a 20% reducer. After sitting on the computer for this whole time, I know GM has a .8241 (3950352) which would get me to 2.707 and I'm currently at 2.65.

I'll keep searching or buy a aftermarket one and change the driven gear on the trans if needed.

And I'm NOT going to pay $236.55 for one!

.8241 Adapter on eBay (http://www.ebay.com/itm/69-70-71-72-73-74-CAMARO-SS-Z28-CHEVELLE-MUNCIE-SPEEDO-ADAPTER-8241-RATIO-YENKO-/320926130350?pt=Vintage_Car_Truck_Parts_Accessorie s&amp;hash=item4ab8b020ae&amp;vxp=mtr)

Kurt

[Postsedan]

I bet Crash has it, and at a fair price <<GRAEMLIN_URL>>/biggthumpup.gif

Dan

[69 Post Sedan]

I found a .800 reducer on GM Parts Direct but I think the shipping is a bit much. <<GRAEMLIN_URL>>/shocked.gif

$411.54 shipping????? (https://secure.gmpartsdirect.com/secure/checkout.3.cfm?CFID=16577766&amp;CFTOKEN=71598954)

Kurt

[crash]

Kurt, I have several in stock, tell which one do you need...

.7333
.8000
.8241
.8653
.9444
.9743
1.0888
1.1111
1.2020
1.3000

Give me a call... Crash

[email protected]

[Postsedan]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Postsedan</div><div class="ubbcode-body">I bet Crash has it, and at a fair price <<GRAEMLIN_URL>>/biggthumpup.gif

Dan </div></div>

I guess I`m a broken record <<GRAEMLIN_URL>>/hmmm.gif

Dan

[396L35]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Postsedan</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Postsedan</div><div class="ubbcode-body">I bet Crash has it, and at a fair price <<GRAEMLIN_URL>>/biggthumpup.gif

Dan </div></div>

I guess I`m a broken record <<GRAEMLIN_URL>>/hmmm.gif

Dan </div></div>

Well Dan I was going to save it until this weekend but since you figured it out on your own I will save my breath... <<GRAEMLIN_URL>>/beers.gif

[Postsedan]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 396L35</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Postsedan</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Postsedan</div><div class="ubbcode-body">I bet Crash has it, and at a fair price <<GRAEMLIN_URL>>/biggthumpup.gif

Dan </div></div>

I guess I`m a broken record <<GRAEMLIN_URL>>/hmmm.gif

Dan </div></div>

Well Dan I was going to save it until this weekend but since you figured it out on your own I will save my breath... <<GRAEMLIN_URL>>/beers.gif </div></div>

Looking forward to seeing you soon.

Dan

[TexasNova]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: crash</div><div class="ubbcode-body">Kurt, I have several in stock, tell which one do you need...

.7333
.8000
.8241
.8653
.9444
.9743
1.0888
1.1111
1.2020
1.3000

Give me a call... Crash

[email protected] </div></div>
Hey crash how do you figure what new speedo gears you need when you change tranny, rear gear, and tires? I know there is a formula but how to you apply it to the gears to recalibrate the speedometer?

[69 Post Sedan]

Ths is the formula I used.

Kurt

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">First off, the way to get the whole system to work right is to make the speed cable rotate 1,000 revolutions for every mile the car is driven. This is done by adjusting the transmission driven to drive gear ratio on the speed cable to match the tire size and rear axle gear ratio of the car.

The first thing to know is how many revolutions the tires make in one mile. To calculate this, measure the diameter of your tires. 26&quot; is about average for F60-15's or 245-60-15's. Multiply this measurement (26&quot;) by pi (3.1416). This will give you the circumference of the tire. Next, divide one mile in inches (63360) by the circumference of tire (63360 divided by (26 x pi)=776). This equals the revs per mile of the tire. Now multiply this number by the rear axle ratio to give you the number of drive shaft revs per mile (776 x 3.91= 3033). Next, divide this number by 1000 (3033 divided by 1000 = 3.033). This is the ratio of transmission driven/drive gears on speedometer cable you will need in this case use a driven gear with 18 teeth and a drive gear with 6 teeth (18 divided by 6 = 3.0). This is the best choice which yields a 1.1% high error which means your odometer unit indicates 1.1% greater than actual or 10.11 for an actual 10.0 miles. If you are using the Ford adapter/reducer (reducer was factory equipped on cars with 3.91, 4.11 or 4.30 rear axle ratios), multiply the number by 0.75 (3.033 x 0.75=2.274). This is the driven/drive ratio you need. A 3.75% error (i.e., a 0.375 mile error in 10 miles) is considered acceptable by design.</div></div>

[enio45]

Damn......i got the slide rule out.....and got the same thing!

[396L35]

Ed are you still carrying that slide rule in your shirt pocket???

[enio45]

I moved it to my back pocket - more room for the pens up front!

[68SSCamaro]

http://www.antiquark.com/sliderule/sim/virtual-slide-rule.html

Martin

[TexasNova]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 69 Post Sedan</div><div class="ubbcode-body">Ths is the formula I used.

Kurt

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">First off, the way to get the whole system to work right is to make the speed cable rotate 1,000 revolutions for every mile the car is driven. This is done by adjusting the transmission driven to drive gear ratio on the speed cable to match the tire size and rear axle gear ratio of the car.

The first thing to know is how many revolutions the tires make in one mile. To calculate this, measure the diameter of your tires. 26&quot; is about average for F60-15's or 245-60-15's. Multiply this measurement (26&quot;) by pi (3.1416). This will give you the circumference of the tire. Next, divide one mile in inches (63360) by the circumference of tire (63360 divided by (26 x pi)=776). This equals the revs per mile of the tire. Now multiply this number by the rear axle ratio to give you the number of drive shaft revs per mile (776 x 3.91= 3033). Next, divide this number by 1000 (3033 divided by 1000 = 3.033). This is the ratio of transmission driven/drive gears on speedometer cable you will need in this case use a driven gear with 18 teeth and a drive gear with 6 teeth (18 divided by 6 = 3.0). This is the best choice which yields a 1.1% high error which means your odometer unit indicates 1.1% greater than actual or 10.11 for an actual 10.0 miles. If you are using the Ford adapter/reducer (reducer was factory equipped on cars with 3.91, 4.11 or 4.30 rear axle ratios), multiply the number by 0.75 (3.033 x 0.75=2.274). This is the driven/drive ratio you need. A 3.75% error (i.e., a 0.375 mile error in 10 miles) is considered acceptable by design.</div></div> </div></div>Okay, my tranny driven/drive gears ratio is 2.817, is there a chart to look at to find the correct or as close at possible gears for calibration?