What speedo gear for 3:23's

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dusterdragracer

dusterdragracer

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I changed to 3:55 gears (new gears stamped 3:55 on them). I have P275-60-15 tires that measure 27 5/8".It is a 833 4 sp.
A chart on Ebay said 33 tooth so I ordered one, and the speedo is 10 MPH fast at 60 MPH.
I contacted the guy that sold it to me, and he said my tire size puts it closer to a 34 tooth.But that 1 tooth is equal to 2 MPH.
Does anybody know the sweet spot? I don't want to keep buying these at $20.00 to guess it.
 
Thanks for the chart, that is the one I used, it is way wrong.If you look at 3:55 with 28" tire, it says 33 tooth. I actually had a 39 tooth in the trans,before I changed gears from when I had 14" tires, and 3:23's and it was closer with the 28" tires and 3:55's. Go figure.
 
I use two charts for comparison from different sources and they are the same. I used the gear in my trans from that chart and it's pretty close.

Most curious. Have you actually measured your tire height or are you going by what the manufacturer says?

Since you are running a 4 speed try and call Brewers Performance and see what they say. They specialize in this type of stuff.
 
bajavan said:
I changed to 3:55 gears (new gears stamped 3:55 on them). I have P275-60-15 tires that measure 27 5/8".It is a 833 4 sp.
A chart on Ebay said 33 tooth so I ordered one, and the speedo is 10 MPH fast at 60 MPH.
I contacted the guy that sold it to me, and he said my tire size puts it closer to a 34 tooth.But that 1 tooth is equal to 2 MPH.
Does anybody know the sweet spot? I don't want to keep buying these at $20.00 to guess it.
Click to expand...


If I understand you right, your speedo reads 70mph when the actual car speed is 60.
If that is correct then you are off by a factor of 1.167 (70/60=1.167)

You need more teeth on the speedo drive gear to slow the speedo down so 33 teeth X 1.17 = 38.6 teeth. So see if you can get a 38 or 39 tooth gear. Doing it this way disregards tire size, gear ratio, etc. Just doesn't matter. I did this using my GPS and my speedo is dead nuts accurate according to Garmin.
 
Wow, that is great info. So the chart is way off.Yes, 27 5/8" is exact ties size, measuered off the car 3 times.And yes, I have a 39 tooth, as I menetioned it seemed prety close. I'll pop it back in.I am thinking a 38 would be perfect. I'll call the trany people.This sight rocks.
 
The chart in the 72 manual only shows tire sizes. "Estimating" that you need a 33-34 tooth, a 34 tooth equates to an H78x14, which shows up as 27.8" tall

Or, a G78 x 15, which shows as 27.7"

YOU CAN NOT measure a tire and calculate tire speed, as tires change under load and rotation.

The 3 places I looked up your tire on the www showed 28", not 27 5/8. Tire size is also gonna change as to the wheel width they are mounted on.

I agree with the above. Figuring percentage change is as good a way as any.

The OTHER factor you may be forgetting, is that not every dad blame car built had an accurate speedo, AND your speedo unit MAY be somewhat out of cal, over the years. You have to remember that a speedo is NOT gear driven (hint --check against the odometer) The speedo is magnetic drive, and age, corrosion, lubricant, or changes in the magnetic drive mechanism can change the calibration.
 
I'm about to stick a 33 tooth gear in my Dart with 3.55's and 28 inch tires. I'll let you all know how it works out.
 
That chart is not way off. I've used it a few times and it's always been real close. I also have a 3.55 gear in my Cuda with 275/60/15 tires and I also tried the 33 tooth first and it was 2 mph off so I put a 34 in and it was right on. The problem is what 273 said, your speedometer calibration is way off, not the chart. I have a buddy with a 74 Challenger that's nearly 20 mph. We can't buy a gear with enough teeth to get it right. You can compensate with yours by using the higher tooth count gear.
 
Thanks guys, as I mentioned, the 39 tooth I was using with the 323 gears,was way closer then the 33 I bought, and it's free because I have it. I think I will take the "the speedo is old and not acurate" view. Hard to believe with a 69,000 original mile car,it would be that much off! But it is old.
I still would like to hear from Brian T after he does his.
 
Age has just as much to do with how mechanical things work as mileage. Things get gummed up from 40 yr. old dried up grease. That can wreak havoc on how well something works. If you aren't the original owner or know the original owner it's hard to know it's history. Someone could have changed the speedometer for one reason or another. The original one in my Cuda only had 45,000 miles on it when the odometer stopped working.
 
bajavan said:
I still would like to hear from Brian T after he does his.
Click to expand...

I put the 33 tooth gear in today, but didn't notice that much difference because I took a 32 tooth out!! The old one was ate up pretty good, with a bunch of missing teeth, which probably explains why my speedo bounced all over the place.

I will try and check it with a buddy pacing me on the highway ASAP.
 
bajavan said:
I changed to 3:55 gears (new gears stamped 3:55 on them). I have P275-60-15 tires that measure 27 5/8".It is a 833 4 sp.
A chart on Ebay said 33 tooth so I ordered one, and the speedo is 10 MPH fast at 60 MPH.
I contacted the guy that sold it to me, and he said my tire size puts it closer to a 34 tooth.But that 1 tooth is equal to 2 MPH.
Does anybody know the sweet spot? I don't want to keep buying these at $20.00 to guess it.
Click to expand...

I have a 8 1/4 3.21 in my dart with 25" tires and 33 tooth puts mine almost dead on with a GPS?
 
I checked mine against a gps program on my phone last night. When the speedo read 40 mph we were going 44 on the gps. When the speedo read 60 we were going 65. I think my tires must be taller than the advertised 28 inches.

My speedometer was closer when I was running the 32 tooth gear. :wack:
 
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