Speedo gear

[bigmanjbmopar]

Ok, so I bought a red 36 tooth gear and had it put in on the rebuild, they took out the green 31 tooth gear.

Story - with the green 31 tooth gear speedo was 5 mph fast so at 65 speedo read 70.


did the math with the calculator. 3.55 gears 26.57" tires comes up as red 36 tooth gear.

Now my speed is mph slow?? so now at 65 speedo reads 60? confused on which way to go up or down? go in between?

Also is it possible to have 3.54 gears in a 8-3/4 with a 489 case? I thought the 3.54 only came in dana 60's?? I ask because doing the hillbilly check with both wheels off the ground and a tyrap on the axle and tape on one wheel I got around 3 rotations from 54 or so drive shaft turns it was close, I just want to believe I have 3.55's someone tell me i just counted slow or something?



from mymopar.com

Speedometer Gear Selection Guide

Overall Tire Diameter​ Axle Ratio 24" 26" 28" 30" 2.76 2538930(Yellow) 2538928(White) 2538926(Red) 2538924(White) 2.94 2538932(Black) 2538930(Yellow) 2538928(White) 2538926(Red) 3.21 2538935(Orange) 2538933(Yellow) 2838931(Green) 2838929(Black) 3.55 2538939(Green) 2538936(Red) 2538933(Yellow) 2538931(Green) 3.91 2538943(Blue) 2538940(Orange) 2538937(White) 2538935(Orange) 4.10 2538945(Yellow) 2538942(White) 2538939(Green) 2538936(Red) 4.30 2538944(Black) 2538941(Red) 2538938(Blue) 4.56 2538945(Yellow) 2538943(Blue) 2538940(Orange) Please Note: These part numbers are for the standard style pinion gear. The last two numbers of the part number is the number of teeth for the gear (Example 2538943 = 43 teeth). Here is an example on how to calculate the wheel diameter to select a speedometer gear to use with the desired ring/pinion gear.Tire Size: 245/60/15245mm/25.4mm = 9.64 Inches Section Width9.64 in. X 60% = 5.79 in. Side Wall Thickness5.79 in. X 2 = 11.57 in. Total Side wall height11.57 in. + 15 in. (Rim Size) = 26.6 in. DiameterSo with this Tire Size and 3.91 Gears, I would use a 40 Tooth Speedometer gear.​

 

[PlumcrazyRR]

Yes, you should be correct. 3.54's came stock in Dana 60's only. The 8 3/4 did not come from the factory with a 3.54.

At 60mph, 1 tooth equals approximately 2-3 mph. Therefore, you would need less teeth which raise the speedometer since it is currently reading low. So you could try out a 34 tooth and that should put you at +/- 2mph or so. I am assuming you are using a GPS or something accurate like that to check your speed?

 

[Cranky]

1 point in gear ratio won't make a hill of beans anyways......

 

[A383Wing]

you will need a 35 tooth gear

 

[PlumcrazyRR]

you will need a 35 tooth gear

Do you think dropping one tooth out of his current setup will give him the 5 mph he is off by right now? In my previous experience (which I am not saying I am a professional) I would say he needs to lose 2 teeth from the speedo gear to make up that 5 mph. Is there something I am missing? I understand from his setup that he has, it probably should be around a 35 tooth, but from his hands on troubleshooting I would say maybe that is not the case here. Just always trying to learn more, that is why I am asking. Thanks.

 

[HT413]

You need to measure from the ground to the axle center, then double that. That is your effective tire diameter, base all calculations off of that. It's a big deal, most tires sag 3/4" to 1", which obviously equates to 1.5" to 2" less effective diameter and will throw your speedo way off.

 

[hunt2elk]

One tooth will change the speedo 1-2 mph. I would try a 34 next if it was mine. The charts never seem to match what they say I need in my cars. It always seems to be a trial and error deal for me.

 

[PlumcrazyRR]

One tooth will change the speedo 1-2 mph. I would try a 34 next if it was mine. The charts never seem to match what they say I need in my cars. It always seems to be a trial and error deal for me.

X2 as I stated above. Glad I am not the only one with that thought process.

 

[bigmanjbmopar]

Well my only options are to try the only 3 choices in between what I have 31,36 which are the 32 black, 33 yellow or 35 orange. Now wondering if I should have my speedo calibrated? I would try a 34 but it's not listed? does anyone have a number and color?

But it will bug me until it's right thanks GPS! lol anyway if 31 was 5 fast with these tires and gears & 36 is 5 slow than 33 seems like the correct choice. that is if a 34 does not exist?

That dam hill of beans gunna make me some coffee out of it!

 

[PlumcrazyRR]

There is a 34 tooth. It is green in color. OE# is 2538934.

 

[A383Wing]

all my cars I have done speedo gears in 1 tooth was about 5 mph....maybe it's me gearing & tire size...but I have not found those charts to be very accurate..I still say he needs a 35 gear..but that's just me

 

[coloradodave]

I'm confused on the method you used to check the rear-end ratio. For 3:55s, ONE rotation of the tire should equal +- 3 1/2 rotations of the driveshaft.

 

[A383Wing]

also remember that it may be 1-2 mph off at low speed, and maybe 7-8 mph off at higher speed....I usually check my speedo accuracy at around 50 mph and gear accordingly

 

[bigmanjbmopar]

Update: My speedo cable was mangled badly against the headers and burned through then when the trany was rebuilt they tried to help it by moving it over and locking it in place that just made the braiding inside mangle up was bad. So after replacing the cable, thanks Dante's for the deal. Got it all within a 1/2 mile through speeds up to 80, so now it is looking good wit the 36 red tooth gear and new cable.

Just got done re doing my cluster, this time took a little more effort and tried something new for me scraped out all the old white paint in the numbers and letters and re painted them in black.