Speedometer gear question

[72hemi]

Not sure if this belongs here or in the transmission section, so I apologize in advance to the mods if I misplaced. Anyways the speedometer in my cuda is off and I want to fix it. I have 3.55 gears and 235 60R 15 tires on the back (approx. 26.1 inches tall) and was wondering what gear I need. I currently have a 32 tooth gear and know that I need one with more teeth, from a couple of charts that I have read it looks like I need a 36 tooth gear. Can anyone verify that before I start ordering speedometer pinion gears.

[GoodysGotaCuda]

Looks like 36 tooth to me as well.

[Changin Gears]

All you have to do calculate the number of teeth required is measure how far off your current speedo is, convert to percent, then get a new gear with the same percentage different teeth.  Example:

Hold your speed at 60mph between 2 mile markers, it requires 54 seconds to travel the distance instead of 60 (you could also use a GPS to get your true speed).  Your speedo is reading 10% low (your really going 66mph).  In this case, if your current gear had 30 teeth you would need one with 27 teeth (10% less) to speed the speedo up. 

I have also used this equation:

Number of Teeth = 259 * (axle ratio/tire diameter)

[72hemi]

Thanks for the formula. From my calculations I need a 35.25 tooth gear, so is it better to agree with the charts and get the 36 tooth gear, or should I round down from my results and get the 35 tooth gear?

[Changin Gears]

If you calculate your true percentage off and chose your gear that way, you are "calibrating" your speedometer.  This is better than a calibration that was done 30+ years ago.

[72hemi]

Perfect, thanks.

[422STROKER]

Where did you get the gears?  I need a 32 according to that formula.  Right now I'm reading too high with the current set-up.

Tom

[RDF]

This thread is right up my alley.....I need a new gear too, seeing as how my speedo says I'm doing 70 when it's actually about 50.....I've got 3.91's

[MEK-Dangerfield]

Where did you get the gears?  I need a 32 according to that formula.  Right now I'm reading too high with the current set-up.

Tom

Tom,

  I got mine from Year One. I got a 33 tooth laying around here someplace, but the rest of what I got are 30 or less.   



   Mike

[422STROKER]

Thanks Mike, I think I got a pretty sweet deal on one from 72Hemi! 

Thanks Again!

Tom

[Changin Gears]

The dealer may still have them, although they would be superseded to newer part numbers.  The original part numbers were 2538926 through 2538945 with the last 2 digits being the number of teeth.