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Changing speedo gear??? #2249644
02/08/17 12:54 PM
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RapidusMaximus Offline OP
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My car came with an original 2.76 gear, as of last weekend it now has a 3.54...anybody know what speedo gear I should now put in to correct the speedometer? The trans is a 904. Thanks up


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Re: Changing speedo gear??? [Re: RapidusMaximus] #2249646
02/08/17 12:59 PM
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Re: Changing speedo gear??? [Re: RapidusMaximus] #2249649
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the tech archive at the top of the page has a speedometer gear chart you may find useful also.
beer

Re: Changing speedo gear??? [Re: moparx] #2249658
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Originally Posted By moparx
the tech archive at the top of the page has a speedometer gear chart you may find useful also.
beer


I agree. I used it to figure out the collection of speedo gears I needed for my junk.

yours will depend on tire size but mine are

235/75/15

2.76 = 25 tooth
3.21 = 29 tooth
3.23 = 29 tooth
3.55 = 32 tooth
3.91 = 35 tooth

325/50/15

2.76 = 26
3.23 = 30
3.91 = 36

Re: Changing speedo gear??? [Re: RapidusMaximus] #2249666
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Re: Changing speedo gear??? [Re: scratchnfotraction] #2249667
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out in the parking lot put a rear valve stem at 6 o'clock then roll the car forward in a straight line till the stem returns to the same position (grab several bricks & your tape) & measure "rollout" then divide Pi into that number for your diameter then as said go to the chart & CNXT (SP) priorly posted a comprehensive chart that is in the tenths on diameter numbers & your gear ratio. Jack the vehicle way up on the DR side to minimize fluid loss.


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Re: Changing speedo gear??? [Re: RapidRobert] #2249686
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Excellent! Not only did I change gears you guys thumped me into remembering that I went from 14" to 15" tires also...sweet! Preferred gear sources?


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Re: Changing speedo gear??? [Re: RapidusMaximus] #2249693
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On Moparts or FABO or Ebay


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Re: Changing speedo gear??? [Re: RapidusMaximus] #2249771
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Re: Changing speedo gear??? [Re: L.R Helbling] #2249836
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CRT has them in stock. and are a board sponsor.

Re: Changing speedo gear??? [Re: scratchnfotraction] #2249867
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Thanks, and looks like my calculations lead to a 33 tooth gear. up


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Re: Changing speedo gear??? [Re: RapidusMaximus] #2249944
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I'd be surprised if you could get a more accurate diameter by measuring roll out and calculating it than just looking up the tire specs. I'd think manufacturing tolerances are way tighter than trying measure roll out.
If you're going to go to that trouble you're better off taking the tire off and actually measuring it.

Re: Changing speedo gear??? [Re: RapidusMaximus] #2250003
02/08/17 11:28 PM
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Originally Posted By RapidusMaximus
Thanks, and looks like my calculations lead to a 33 tooth gear. up



That would be the same one that is on my car. That's the yellow speedo gear part no. 2538933. thumbs

Re: Changing speedo gear??? [Re: L.R Helbling] #2250018
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I would think that "rollout" would be the most accurate measurement cuz it would be exactly what the # of turns is that will be imparted to the drivetrain & something I remember from drag racers way back is that the tire might even grow some at speed. If you grabbed 2 bricks, your tape and a calculator & have ~8 ft of space in front of your car I think you could knock it out in 5 to 10 minutes. If you do it you might compare it to the tire listed dia & we can see for sure.


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Re: Changing speedo gear??? [Re: RapidusMaximus] #2250193
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For one of our math whizzes out there.

Say you have a 275-60-15 inch tire mounted and aired up.

Laying flat on the ground, let's say measures 28 inch diameter.

Now mount the wheel on the car, lower to where is is sitting on the pavement.

As tire will flatten out a bit from the weight of the car, perhaps it is now 27 inches from the pavement to the top of the tire.

How does this effect the rolling circumference of the tire?

If no flattening, 1 full revolution would move the auto 28 inches times pi of 3.1416 equal 88.9 inches.

How far will the car move forward in one revolution allowing for 1 inch less diameter due to this flattening?

Often have wondered about this and how it effects odometer calculations.

Re: Changing speedo gear??? [Re: RapidusMaximus] #2250194
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Sxrxrnr- the diameter of the tire is not what is needed here - what you need is the "loaded tire radius".
If you measure from the exact center of the axle flange, hub cap, wheel cover - to the ground and do the correct calculations you will end up with the same information without the bricks or moving the vehicle. Either method does not take in to consideration the growth of the tire as speed increases but I doubt that growth is much at speeds less than 70 mph.

Re: Changing speedo gear??? [Re: A727Tflite] #2250222
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Thanks for all the replies guys, appreciate the input, I took the easy way out and used the tire size/gear ratio and just rolled with that. With the current speedo gear being calibrated for 2.76 and 14" tires you can imagine how far the speedometer is off! eek grin


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1974 Dodge P/U Long Bed Stepside 318
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Re: Changing speedo gear??? [Re: RapidusMaximus] #2250224
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Rapidus,

Say it ain't so!! You calibrated your speedo gear on 14" tires!!!

No wonder you are way off. shock

Re: Changing speedo gear??? [Re: RapidusMaximus] #2250249
02/09/17 11:44 AM
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It is easy to calculate exactly the speedometer gear you need but you need to know the axle ratio and the tire size. The previous replies are correct about the importance of using the loaded tire size - also called the rolling diameter. The rolling tire diameter is about 3% smaller than the static (unloaded) diameter.

Tire Dimensions
Tire Upgrades

Re: Changing speedo gear??? [Re: A727Tflite] #2250458
02/09/17 05:11 PM
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Originally Posted By Transman
Sxrxrnr- the diameter of the tire is not what is needed here - what you need is the "loaded tire radius".


I disagree. The rollout (which computes to true diameter) will differ from the squatted radius.


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Re: Changing speedo gear??? [Re: RapidusMaximus] #2250759
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You can disagree all you want - what do you have to back it up?

Re: Changing speedo gear??? [Re: A727Tflite] #2250967
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i believe the roll out will be what you need to be the most accurate. to back this up, just take a tank "tread". in use, it has a super short "loaded radius". if you put it in a circle, it would be pretty tall. un-link it, and it is a set length, never changing length in any shape. i believe a tire acts the same. the tank tread will roll a set distance no matter the shape because of the "length" of the tread. it takes "x" time to cover "x" distance [roll out, or circumference] at "x" speed. we are not talking a top fuel slick growing at speed by any measure.
beer

Re: Changing speedo gear??? [Re: A727Tflite] #2251039
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Re: Changing speedo gear??? [Re: CRT] #2251084
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Speedo gear coming from CRT...thanks John up


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Re: Changing speedo gear??? [Re: A727Tflite] #2251106
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Originally Posted By Transman
You can disagree all you want - what do you have to back it up?


My actual measurements...try it yourself.


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Re: Changing speedo gear??? [Re: RapidusMaximus] #2251189
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Still wondering why anyone thinks measuring the tire is more accurate than the manufacturer specs that are readily available?

Re: Changing speedo gear??? [Re: BDW] #2251235
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Example, the manufacturer of my 235/70R15's says they are 28" in diameter but the rollout is 84" which equates to a diameter of 26.73".


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Re: Changing speedo gear??? [Re: John_Kunkel] #2251268
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Originally Posted By John_Kunkel

Example, the manufacturer of my 235/70R15's says they are 28" in diameter but the rollout is 84" which equates to a diameter of 26.73".


iagree

I agree with John, my way is to use gps and install the proper speedometer gear... up

Re: Changing speedo gear??? [Re: STROKIE] #2251466
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Originally Posted By STROKIE
Originally Posted By John_Kunkel

Example, the manufacturer of my 235/70R15's says they are 28" in diameter but the rollout is 84" which equates to a diameter of 26.73".


iagree

I agree with John, my way is to use gps and install the proper speedometer gear... up


John exactly illustrates my point in asking the question of math whizzes. Also completely agree with gps recommendations as an exact time saving tool.

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Re: Changing speedo gear??? [Re: Sxrxrnr] #2251743
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At the risk of repeating myself, I think it's ill-advised to change a speedo pinion based on a GPS speed readout; the odometer is the best indicator of correct/incorrect pinion, not the speedometer. If the odometer is correct, then any speedo error is in the head, not the pinion.


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Re: Changing speedo gear??? [Re: John_Kunkel] #2251799
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Originally Posted By John_Kunkel

At the risk of repeating myself, I think it's ill-advised to change a speedo pinion based on a GPS speed readout; the odometer is the best indicator of correct/incorrect pinion, not the speedometer. If the odometer is correct, then any speedo error is in the head, not the pinion.


Thank you John, for the clarification.
That's why I like Moparts.com

Re: Changing speedo gear??? [Re: STROKIE] #2252816
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Originally Posted By STROKIE
Originally Posted By John_Kunkel

At the risk of repeating myself, I think it's ill-advised to change a speedo pinion based on a GPS speed readout; the odometer is the best indicator of correct/incorrect pinion, not the speedometer. If the odometer is correct, then any speedo error is in the head, not the pinion.


Thank you John, for the clarification.
That's why I like Moparts.com


The above illustrated app if you forego the 'free' version and step up to the paid version(2 bucks was what I paid) you get among other niceties, includes odometer support and a history/log file. .

The cost of a small order of fries at Macdonald's.

What could be simpler and more effective?

Re: Changing speedo gear??? [Re: Sxrxrnr] #2252912
02/13/17 01:32 PM
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Agreed, with the right speedo gear the ODO will automatically be corrected & most always then the magnetic gismo speedo will also be correct & if not then the speedo head in that rare case will need to be sent to a speedo specialty shop to be recalibrated (or another one at a boneyard may be cheaper).


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Re: Changing speedo gear??? [Re: Sxrxrnr] #2252992
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Originally Posted By Sxrxrnr


The above illustrated app if you forego the 'free' version and step up to the paid version(2 bucks was what I paid) you get among other niceties, includes odometer support and a history/log file.


The app Speed View is totally free and provides an odometer reading.

https://play.google.com/store/apps/details?id=com.codesector.speedview.free&hl=en


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Re: Changing speedo gear??? [Re: RapidusMaximus] #2253149
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I think I'd be more worried about the quality of the tire than the speedo accuracy if the vendor can't make a tire that's not 1" off from a pretty standard spec.....

Re: Changing speedo gear??? [Re: BDW] #2253588
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The same size tire from the same manufacturer can be different in diameter due to coming from different molds and the diameter of the same tire will vary with the width of the wheel it's mounted on.


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